wasm: add QWasmOffscreenSurface

This no-op implementation is sufficient to support OpenGL cleanup
use cases, where the OpenGL context needs to be made current at times
where we don't have a window available.

A specific requirement on WebAssembly is that the context is tied
to one specific screen; which is an extra requirement on QWasmOffscreenSurface,
compared to the other platforms.

Task-number: QTBUG-75463
Change-Id: Ie3658cb235bf342be66f19dfe981e3a56a90e1b6
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

src/plugins/platforms/wasm/qwasmintegration.cpp

@@ -35,6 +35,7 @@
 #include "qwasmtheme.h"
 #include "qwasmclipboard.h"
 #include "qwasmservices.h"
+#include "qwasmoffscreensurface.h"
 
 #include "qwasmwindow.h"
 #ifndef QT_NO_OPENGL
@@ -188,6 +189,11 @@ QPlatformOpenGLContext *QWasmIntegration::createPlatformOpenGLContext(QOpenGLCon
 }
 #endif
 
+QPlatformOffscreenSurface *QWasmIntegration::createPlatformOffscreenSurface(QOffscreenSurface *surface) const
+{
+    return new QWasmOffscrenSurface(surface);
+}
+
 QPlatformFontDatabase *QWasmIntegration::fontDatabase() const
 {
     if (m_fontDb == nullptr)

src/plugins/platforms/wasm/qwasmintegration.h

@@ -65,6 +65,7 @@ public:
 #ifndef QT_NO_OPENGL
     QPlatformOpenGLContext *createPlatformOpenGLContext(QOpenGLContext *context) const override;
 #endif
+    QPlatformOffscreenSurface *createPlatformOffscreenSurface(QOffscreenSurface *surface) const override;
     QPlatformFontDatabase *fontDatabase() const override;
     QAbstractEventDispatcher *createEventDispatcher() const override;
     QVariant styleHint(QPlatformIntegration::StyleHint hint) const override;

src/plugins/platforms/wasm/qwasmoffscreensurface.cpp

@@ -0,0 +1,41 @@
+/****************************************************************************
+**
+** Copyright (C) 2019 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the plugins of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 or (at your option) any later version
+** approved by the KDE Free Qt Foundation. The licenses are as published by
+** the Free Software Foundation and appearing in the file LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include "qwasmoffscreensurface.h"
+
+QWasmOffscrenSurface::QWasmOffscrenSurface(QOffscreenSurface *offscreenSurface)
+    :QPlatformOffscreenSurface(offscreenSurface)
+{
+
+}
+
+QWasmOffscrenSurface::~QWasmOffscrenSurface()
+{
+
+}

src/plugins/platforms/wasm/qwasmoffscreensurface.h

@@ -0,0 +1,49 @@
+/****************************************************************************
+**
+** Copyright (C) 2019 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the plugins of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 or (at your option) any later version
+** approved by the KDE Free Qt Foundation. The licenses are as published by
+** the Free Software Foundation and appearing in the file LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#ifndef QWASMOFFSCREENSURFACE_H
+#define QWASMOFFSCREENSURFACE_H
+
+#include <qpa/qplatformoffscreensurface.h>
+
+QT_BEGIN_NAMESPACE
+
+class QOffscreenSurface;
+class QWasmOffscrenSurface : public QPlatformOffscreenSurface
+{
+public:
+    explicit QWasmOffscrenSurface(QOffscreenSurface *offscreenSurface);
+    ~QWasmOffscrenSurface();
+private:
+
+};
+
+QT_END_NAMESPACE
+
+#endif

src/plugins/platforms/wasm/wasm.pro

@@ -20,7 +20,8 @@ SOURCES = \
     qwasmopenglcontext.cpp \
     qwasmtheme.cpp \
     qwasmclipboard.cpp \
-    qwasmservices.cpp
+    qwasmservices.cpp \
+    qwasmoffscreensurface.cpp
 
 HEADERS = \
     qwasmintegration.h \
@@ -35,7 +36,8 @@ HEADERS = \
     qwasmopenglcontext.h \
     qwasmtheme.h \
     qwasmclipboard.h \
-    qwasmservices.h
+    qwasmservices.h \
+    qwasmoffscreensurface.h
 
 wasmfonts.files = \
     ../../../3rdparty/wasm/Vera.ttf \