The ubershader has some corner cases where it can't be used, in
particular when the child is massively larger than the repeat node and
the repeat node is used to clip lots of the source.
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.
They're done using the pattern shader.
The pattern shader now gained a stack where vec4's can be pushed and
popped back later, which allows storing the position before computing
the new position inside the repeat node's child.
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.
Add a bunch of inline functions for graphene_rectangle_t.
We use those quite extensively in tight loops so making them as fast as
possible via inlining has massive benefits.
The current render-heavy benchmark I am playing (th paris-30k in node-editor)
went from 49fps to 85fps on my AMD.
