Make the generator generate calls for the correct glBindAttribLocation()
calls.
Usually this was done correctly, but we can't rely on it. So do it
explicitly.
This way we can toggle opacity handling on/off.
THe shader slowly turns into a fancy texture op - but I don't want to
rename it to "fancytexture" just yet.
A variation is a #define/specialization constant that every shader can
use to specialize itself as it sees fit.
This commit adds the infrastrcture, future commits will add
implementations.
There are various places where the alpha is implicitly assumed to be
handled, so just handle it.
As a bonus, this simplifies a bunch of code and makes the texture node
rendering work with alpha.
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.
