metal: assertion when scene has refraction and all effects are disabled

[Mathias Agopian]

To reproduce:

gltf_viewer -a metal  TransmissionTest.glb

Then either run the automation tests or disable post-processing, MSAA and SSAO.
This will result in this assertion:

-[MTLDebugRenderCommandEncoder validateCommonDrawErrors:]:5252: failed assertion `Draw Errors Validation
Fragment Function(blitterFrag): Shader reads texture (sourceColor[0]) whose usage (0x04) doesn't specify MTLTextureUsageShaderRead (0x01)