Add OpenGL-only notes to the simple materials

Change-Id: I4b7f9ef379f2641da9c06572daa5ba41b03dd994
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

src/quick/scenegraph/util/qsgflatcolormaterial.cpp

@@ -125,6 +125,9 @@ void FlatColorMaterialShader::initialize()
     \inmodule QtQuick
     \ingroup qtquick-scenegraph-materials
 
+    \warning This utility class is only functional when running with the OpenGL
+    backend of the Qt Quick scenegraph.
+
     The flat color material will fill every pixel in a geometry using
     a solid color. The color can contain transparency.
 

src/quick/scenegraph/util/qsgsimplematerial.cpp

@@ -41,11 +41,14 @@
     \class QSGSimpleMaterialShader
 
     \brief The QSGSimpleMaterialShader class provides a convenient way of
-    building custom materials for the scene graph.
+    building custom OpenGL-based materials for the scene graph.
 
     \inmodule QtQuick
     \ingroup qtquick-scenegraph-materials
 
+    \warning This utility class is only functional when running with the OpenGL
+    backend of the Qt Quick scenegraph.
+
     Where the QSGMaterial and QSGMaterialShader API requires a bit of
     boilerplate code to create a functioning material, the
     QSGSimpleMaterialShader tries to hide some of this through the use

src/quick/scenegraph/util/qsgtexturematerial.cpp

@@ -127,6 +127,9 @@ void QSGOpaqueTextureMaterialShader::updateState(const RenderState &state, QSGMa
     \inmodule QtQuick
     \ingroup qtquick-scenegraph-materials
 
+    \warning This utility class is only functional when running with the OpenGL
+    backend of the Qt Quick scenegraph.
+
     The opaque textured material will fill every pixel in a geometry with
     the supplied texture. The material does not respect the opacity of the
     QSGMaterialShader::RenderState, so opacity nodes in the parent chain
@@ -321,6 +324,9 @@ int QSGOpaqueTextureMaterial::compare(const QSGMaterial *o) const
     \inmodule QtQuick
     \ingroup qtquick-scenegraph-materials
 
+    \warning This utility class is only functional when running with the OpenGL
+    backend of the Qt Quick scenegraph.
+
     The textured material will fill every pixel in a geometry with
     the supplied texture.
 

src/quick/scenegraph/util/qsgvertexcolormaterial.cpp

@@ -107,6 +107,9 @@ void QSGVertexColorMaterialShader::initialize()
     \inmodule QtQuick
     \ingroup qtquick-scenegraph-materials
 
+    \warning This utility class is only functional when running with the OpenGL
+    backend of the Qt Quick scenegraph.
+
     The vertex color material will give each vertex in a geometry a color. Pixels between
     vertices will be linearly interpolated. The colors can contain transparency.