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.
