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.
There's a well hidden line in the spec that says in
https://registry.khronos.org/vulkan/specs/1.3/html/chap15.html#interfaces-resources-descset
If the combined image sampler enables sampler Y′CBCR conversion,
it **must** be indexed only by constant integral expressions when
aggregated into arrays in shader code, irrespective of the
shaderSampledImageArrayDynamicIndexing feature.
So we'll use the same trick that we use for old GL here and do an
if dance that gives us dynamically uniform expressions.
Define an array with a compile-time-constant variable size for the
immutable samplers.
A bunch of work is necessary to ensure that at least one element is in
the sampler array, because the GLSL code
sampler2D immutable_textures[0];
is invalid.
This allows having different layouts sothat we can support immutable
samplers, whcih are required for multiplane and YUV formats.
We don't use them yet.
Due to GLES and old GL not allowing non-constant texture array
lookups,we need to turn the array lookup into a big switch statementin
those versions, and that requires putting the texture() call into that
switch.
But with that trick, we can use texture IDs in GLSL.
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.
