The env var allows skipping various optimizations in the GPU shader.
This is useful for testing during development when trying to figure
out how to make a renderer as fast as possible.
We could also use it to enable/disable optimizations depending on GL
version or so, but I didn't think about that too much yet.
Frames now carry a timestamp for when they are used.
This is mainly intended to attach timestamps to cached items (textures
or glyphs), but it could in theory also be used when profiling.
We use wallclock time here, not server time, because it's cheaper and
because we're more intereseted in the local machine we're rendering on.
For now, it just renders using cairo, uploads the result to the GPU,
blits it onto the framebuffer and then is happy.
But it can do that using Vulkan and using GL (no idea which version).
The most important thing still missing is shaders.
It also has a bunch of copy/paste from the Vulkan renderer that isn't
used yet.
But I didn't want to rip it out and then try to copy it back later
