Due to GLES and old GL not allowing non-constant texture array
lookups,we need to turn the array lookup into a big switch statementin
those versions, and that requires putting the texture() call into that
switch.
But with that trick, we can use texture IDs in GLSL.
... and use it for glyphs.
The name is a slight variation of the "coloring" name from the GL
renderer.
The functionality is exactly what the "glyph" shader from the Vulkan
renderer does.
1. Compute the fwidth() twice with offset offsets
That way, we avoid glitches at the boundary between 0.0 and 1.0,
because by offsetting it by 0.5, that boundary goes away.
Then we take the min() of both which gives us the one we care about.
2. Set the gradient to repeating
By doing that, we don't get values at the 0.0/1.0 boundary clamped,
but things smoothly transition.
This smoothes the line at that boundary and makes it look just like
every other line.
Pretty much a copy of the Vulkan border shader.
A notable change is that the input arguments are changed, because GL
gets confused if you put a mat4 at the end.
This heaves over an inital chunk of code from the Vulkan renderer to
execute shaders.
The only shader that exists for now is a shader that draws a single
texture.
We use that to replace the blit op we were doing before.
