Proposal: split up all code shared between pipelines.

[mintlu8]

Why

The pipelined rendering system that bevy currently have is great but feels almost impossible to learn. This is due to the fact a lot of code is shared between multiple pipelines, gated behind if statements and wgsl #ifdefs, making them difficult to read or modify.

Proposed Solution

• Remove all wgsl definitions regarding pipelines, split up shader code if needed.
• Split up specialize on Material into 3 functions for each pipeline.
  • specialize
  • prepass_specialize
  • deferred_specialize