The GLSL to MSL conversion process uses a passed-in sampler state for
the `iChannel0` parameter and we weren't providing it. This magically
worked on Apple Silicon for unknown reasons but failed on Intel GPUs.

In normal, hand-written MSL, we'd explicitly create the sampler state as
a normal variable (we do this in `shaders.metal` already!), but the
Shadertoy conversion stuff doesn't do this, probably because the exact
sampler parameters can't be safely known.

This fixes a Metal validation error when using custom shaders:

```
-[MTLDebugRenderCommandEncoder validateCommonDrawErrors:]:5970: failed 
assertion `Draw Errors Validation Fragment Function(main0): missing Sampler 
binding at index 0 for iChannel0Smplr[0].
```

This sets up for a couple improvments (see TODO comments) and also sets
the glyph atlas textures to nearest neighbor sampling since we can do
that now that we never scale glyphs.

BPTC is required to be available OpenGL >= 4.2 and our minimum is 4.3 so
this is safe in terms of support. I tested briefly in a VM and didn't
encounter any problems so this should just be a complete win.

(Note: texture data is already automatically compressed on Metal)

This commit is very large, representing about a month of work with many
interdependent changes that don't separate cleanly in to atomic commits.

The main change here is unifying the renderer logic to a single generic
renderer, implemented on top of an abstraction layer over OpenGL/Metal.

I'll write a more complete summary of the changes in the description of
the PR.

otherwise images will be too dark when scaled

https://ziglang.org/download/0.14.0/release-notes.html#Calling-Convention-Enhancements-and-setAlignStack-Replaced

This adds a CI test to ensure that all Zig files are properly formatted.
This avoids unrelated diff noise in future PRs.