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.
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.
Introduce a new GskGpuImageDescriptors object that tracks descriptors
for a set of images that can be managed by the GPU.
Then have each GskGpuShaderOp just reference the descriptors object they are
using, so that the coe can set things up properly.
To reference an image, the ops now just reference their descriptor -
which is the uint32 we've been sending to the shaders since forever.
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.
