`render/wgpu`: Drop `naga_oil` in favor of manual string manipulation

[torokati44]

One fewer dependency. We've only used the very basics of its capabilities lately anyway. Also, so we're not stuck waiting for the likes of https://github.com/bevyengine/naga_oil/pull/87.

Unblocks https://github.com/ruffle-rs/ruffle/pull/16168, and the update to egui master (temporarily), thereby the fixing of #15836.

TODO:

• sort out vertex colliding entry point names...?
• pipeline constants for early_saturate
  • After the switch to wgpu 0.20
  • Currently solved with a simple string substitution, the compiler will probably optimize out the dead piece of code.
• investigate effect on shader compilation times (startup)

[torokati44]

investigate effect on shader compilation times (startup)

Do you think you could take a look at this, @adrian17 ? You're the only one I remember having experienced any sort of noticeable startup delay...