The GL branch should eventually call into gdk_gl_context_get_scale(),
which is what checks for GDK_DEBUG=gl-fractional; whereas the Vulkan
branch needs no change.
Use glDrawArraysInstancedBaseInstance() to draw. (Yay for GL naming.)
That allows setting up the offset in the vertex array without having to
glVertexAttribPointer() everything again.
However, this is only supported since GL 4.2 and not at all in stock GLES,
so we need to have code that can work without it.
Fortunately, it is mandatory in Vulkan, so every recent GPU supports it.
And if that GPU has a proper driver, it will also expose the GL extension
for it.
(Hint: You can check https://opengles.gpuinfo.org/listextensions.php for
how many proper drivers exist outside of Mesa.)
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.
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
