rhi: Add a feature flag for readback format support

Indicate that doing a QRhiResourceUpdateBatch::readBackTexture() for
texture formats other than RGBA/BGRA is not necessarily supported at
run time.

Change-Id: Ie9ca9546a3af9bff142b875f1ecf26bf26bcc442
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

src/gui/rhi/qrhi.cpp

@@ -596,6 +596,18 @@ Q_LOGGING_CATEGORY(QRHI_LOG_INFO, "qt.rhi.general")
     type attributes will be broken. In practice this feature will be unsupported
     with OpenGL ES 2.0 and OpenGL 2.x, while it will likely be supported
     everywhere else.
+
+    \value ScreenSpaceDerivatives Indicates that functions such as dFdx(),
+    dFdy(), and fwidth() are supported in shaders.
+
+    \value ReadBackAnyTextureFormat Indicates that reading back texture
+    contents can be expected to work for any QRhiTexture::Format. When reported
+    as false, which will typically happen with OpenGL, only the formats
+    QRhiTexture::RGBA8 and QRhiTexture::BGRA8 are guaranteed to be supported
+    for readbacks. In addition, with OpenGL, but not OpenGL ES, reading back
+    the 1 byte per component formats QRhiTexture::R8 and
+    QRhiTexture::RED_OR_ALPHA8 are supported as well. Backends other than
+    OpenGL can be expected to return true for this feature.
  */
 
 /*!

src/gui/rhi/qrhi_p.h

@@ -1457,7 +1457,8 @@ public:
         TexelFetch,
         RenderToNonBaseMipLevel,
         UIntAttributes,
-        ScreenSpaceDerivatives
+        ScreenSpaceDerivatives,
+        ReadBackAnyTextureFormat
     };
 
     enum BeginFrameFlag {

src/gui/rhi/qrhid3d11.cpp

@@ -481,6 +481,8 @@ bool QRhiD3D11::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::ScreenSpaceDerivatives:
         return true;
+    case QRhi::ReadBackAnyTextureFormat:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;

src/gui/rhi/qrhigles2.cpp

@@ -875,6 +875,8 @@ bool QRhiGles2::isFeatureSupported(QRhi::Feature feature) const
         return caps.uintAttributes;
     case QRhi::ScreenSpaceDerivatives:
         return caps.screenSpaceDerivatives;
+    case QRhi::ReadBackAnyTextureFormat:
+        return false;
     default:
         Q_UNREACHABLE();
         return false;
@@ -2400,6 +2402,7 @@ void QRhiGles2::executeCommandBuffer(QRhiCommandBuffer *cb)
             const int h = result->pixelSize.height();
             if (mipLevel == 0 || caps.nonBaseLevelFramebufferTexture) {
                 // With GLES, GL_RGBA is the only mandated readback format, so stick with it.
+                // (and that's why we return false for the ReadBackAnyTextureFormat feature)
                 if (result->format == QRhiTexture::R8 || result->format == QRhiTexture::RED_OR_ALPHA8) {
                     result->data.resize(w * h);
                     QByteArray tmpBuf;

src/gui/rhi/qrhimetal.mm

@@ -570,6 +570,8 @@ bool QRhiMetal::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::ScreenSpaceDerivatives:
         return true;
+    case QRhi::ReadBackAnyTextureFormat:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;

src/gui/rhi/qrhivulkan.cpp

@@ -4052,6 +4052,8 @@ bool QRhiVulkan::isFeatureSupported(QRhi::Feature feature) const
         return true;
     case QRhi::ScreenSpaceDerivatives:
         return true;
+    case QRhi::ReadBackAnyTextureFormat:
+        return true;
     default:
         Q_UNREACHABLE();
         return false;