This shader can take over from the ubershader. And it can be used
instead of launching the ubershader when no offscreens are necessary.
Also includes an optimization that uses the colorize shader when
appropriate.
Add a GSK_GPU_IMAGE_STRAIGHT_ALPHA and use it for images that have
straight alpha.
Make sure those images get passed through a premultiplying pass with
the new straight alpha shader.
Also remove the old Postprocess flags from the Vulkan image that were a
leftover from copying that code from the old Vulkan renderer.
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.
Pretty much a copy of the Vulkan border shader.
A notable change is that the input arguments are changed, because GL
gets confused if you put a mat4 at the end.
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.
