Partial Depth-GrabPass support for WebGPU

[mvaligursky]

• depth grabpass texture only works when render target / framebuffer is single sampled, by copying out a depth texture. For multisampled case, WebGPU does not allow us to resolve / copy the multisampled depth, and a manual resolve using custom shader will need to be implemented in the future
• added a shared chunk to graphics, which gets included in all fragment shaders, for global built-in functions. Currently contains a function to generate texture coordinates for sampling from grab pass, and this handles the upside down case for WebGPU.