The main reason here is that we want to not fail when the texture size
is larger than the supported GpuImage size.
When that happens, for now we just fallback slowly - ulitmately to
drawing with Cairo, which is going to be clipped.
Because GL flips its shit sometimes (ie when it's the framebuffer),
pass the height of the target as the flip variable, so commands
that need to operate on the pixels can flip the y axis around this value.
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.
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
