Instead of having renderer API to wait for any number of frames, just
have gsk_gpu_frame_wait() to wait for a single frame.
This unifies behavior on Vulkan and GL, because unlike Vulkan, GL does
not allow waiting for multiple fences.
To make up for it, we replace waiting for multiple frames with finding
the frame with the earliest timestamp and waiting for that one.
Also implement wait() for GL.
If shaders don't support nonuniform indexing, we emulate it via if/else
ladders (or switch ladders) which get inlined by the GLSL compiles and
massively blow up the code.
And that makes compilation of the shaders take minutes and results in
shader code that isn't necessarily faster.
So we disable it on GL entirely and on Vulkan if the required features
aren't available.
As it's only an optimization and does not fall back to Cairo anymore,
this should be fine.
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.)
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
