eglfs: fix cleanup when more than one tlw was used

There is nothing guaranteeing there is a context current when the
backingstore dtor is invoked for widget windows that do not contain
render-to-texture widgets. In some cases the eglfs compositor's
context is still current, while in other cases (esp. when having
popups and other top-levels) there is none.

To prevent not releasing the backingstore texture and the incorrect
warning about incorrect context, make the correct context current via
an offscreen surface, when necessary.

Change-Id: Id8257650c1ec8cf96910a4f285b779419c3558a8
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>

src/platformsupport/platformcompositor/qopenglcompositorbackingstore.cpp

@@ -34,6 +34,7 @@
 #include <QtGui/QOpenGLContext>
 #include <QtGui/QWindow>
 #include <QtGui/QPainter>
+#include <QtGui/QOffscreenSurface>
 #include <qpa/qplatformbackingstore.h>
 #include <private/qwindow_p.h>
 
@@ -82,13 +83,28 @@ QOpenGLCompositorBackingStore::~QOpenGLCompositorBackingStore()
 {
     if (m_bsTexture) {
         QOpenGLContext *ctx = QOpenGLContext::currentContext();
+        // With render-to-texture-widgets QWidget makes sure the TLW's shareContext() is
+        // made current before destroying backingstores. That is however not the case for
+        // windows with regular widgets only.
+        QScopedPointer<QOffscreenSurface> tempSurface;
+        if (!ctx) {
+            ctx = QOpenGLCompositor::instance()->context();
+            tempSurface.reset(new QOffscreenSurface);
+            tempSurface->setFormat(ctx->format());
+            tempSurface->create();
+            ctx->makeCurrent(tempSurface.data());
+        }
+
         if (ctx && m_bsTextureContext && ctx->shareGroup() == m_bsTextureContext->shareGroup())
             glDeleteTextures(1, &m_bsTexture);
         else
             qWarning("QOpenGLCompositorBackingStore: Texture is not valid in the current context");
+
+        if (tempSurface)
+            ctx->doneCurrent();
     }
 
-    delete m_textures;
+    delete m_textures; // this does not actually own any GL resources
 }
 
 QPaintDevice *QOpenGLCompositorBackingStore::paintDevice()
@@ -254,6 +270,7 @@ void QOpenGLCompositorBackingStore::resize(const QSize &size, const QRegion &sta
     if (m_bsTexture) {
         glDeleteTextures(1, &m_bsTexture);
         m_bsTexture = 0;
+        m_bsTextureContext = Q_NULLPTR;
     }
 }