Use the static TexStorage2|3D funcs in es builds

When building with -opengl es2 and having ES 3.0 or newer, TexStorage must resolve
directly to the functions.

Other paths (desktop, ES when using dynamic on Windows) are already covered below.

Change-Id: If214578bf23547d6f66a17bc999e9fabf97ba770
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/gui/opengl/qopengltexturehelper.cpp

@@ -200,8 +200,13 @@ QOpenGLTextureHelper::QOpenGLTextureHelper(QOpenGLContext *context)
     TexImage2DMultisample = 0;
 
     // OpenGL 4.2
+#ifdef QT_OPENGL_ES_3
+    TexStorage3D = ::glTexStorage3D;
+    TexStorage2D = ::glTexStorage2D;
+#else
     TexStorage3D = 0;
     TexStorage2D = 0;
+#endif
     TexStorage1D = 0;
 
     // OpenGL 4.3