Scene3D as texture provider

Providing a QSGTextureProvider from Scene3D should allow it to be used by ShaderEffect (including QtGraphicalEffects) without an additional FBO.

There are a few things that make this potentially more difficult:

• vertical flip (the texture used is vertically flipped from what ShaderEffect expects)
• no premultiplied alpha (currently Scene3D handles this in the shader; maybe a texture provider would only work when Scene3D had opaque content?)

This might also require the texture provided to be a QSGDynamicTexture subclass for updates to work properly.