Provide functionality equivalent to GL_EXT_shader_framebuffer_fetch

Upon my reading of the spec, it seems like I cannot use subpassLoad if I've already written to the subpass input attachment within this subpass.

This is a far reduced featureset compared to OpenGL ES's GL_EXT_shader_framebuffer_fetch or GL_ARM_pixel_local_storage.

Use Case

Multi-Layer Alpha Blending on Mobile GPUs.

The funny thing is that I can implement this on Dekstop GPUs same way I would with Vulkan (pixel shader interlock/ordering) via storage images.

But for Mobile+Intel I am able (and forced to, due to mobile GPUs lacking pixel shader interlock/ordering) to implement a turbo-optimized variant via EXT_shader_framebuffer_fetch having the images as FBO attachments instead of storage images.

Even Metal API has this feature, so its a shame that Vulkan doesn't have it yet.