This exposes interfaces to render glyphs into a texture atlas. The main changes are:
* Methods to plumb raw Metal GPU resources (device, texture, etc) into piet-gpu-hal objects.
* A new glyph_render API specialized to rendering glyphs. This is basically the same as just painting to a canvas, but will allow better caching (and has more direct access to fonts, bypassing the Piet font type which is underdeveloped).
* Ability to render to A8 target in addition to RGBA.
WIP, there are some rough edges, not least of which is that the image format changes are only on mac and cause compile errors elsewhere.
Make max workgroup size 256 and respect LG_WG_FACTOR.
Because the monoid scans only support a height of 2, this will reduce
the maximum scene complexity we can render. But it also increases
compatibility. Supporting larger scans is a TODO.
There's a bit of reorganizing as well. Shader stages are made available
from piet-gpu to the test rig, config is now a proper structure
(marshaled with bytemuck).
This commit just has the transform stage, which is a simple monoid scan
of affine transforms.
Progress toward #119
