Instead of using an enum, use a usual custom class struct like we use
for GskGpuOp.
As a side effect of that refactoring, the display gained a hash table
for textures where we can't use the render data because the texture is
used in multiple renderers.
The goal here is that a texture is always cached and we can ensure that
there is a 1:1 relation between textures and their GskGpuImage. This is
important in particular for external textures - like dmabufs - where we
absolutely don't want 2 images with 2 device memories, and where we use
toggle references to keep them alive.
This adds GSK_GPU_IMAGE_CAN_MIPMAP and GSK_GPU_IMAGE_MIPMAP flags and
support to ensure_image() and image creation functions for creating a
mipmapped image.
Mipmaps are created using the new mipmap op that uses
glGenerateMipmap() on GL and equivalent blit ops on Vulkan.
This is then used to ensure the image is mipmapped when rendering it
with a texture-scale node.
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
