Some maps are used for read only and do not require uploading contents
back to the GPU afterwards. In other cases, we can often upload less than
the fully allocated buffer size.
This ensures both that we signal a semaphore for a dmabuf when we export
an image and that we import semaphores for dmabufs and wait on them.
Fixes Vulkan node-editor displaying the Vulkan renderer in the sidebar.
Make gsk_renderer_render_texture() create a dmabuf texture if that is
possible.
If it isn't (ie if we're not on Linux or if dmabufs are otherwise not
working) fall back to the previous code of creating a memory texture.
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
