vulkan: Add premultiply step to texture upload

vulkan: Add premultiply step to texture upload

When a GdkMemoryFormat is not supported natively and there's
postprocessing required, add a way to mark a VulkanImage as such via the
new postprocess flags.

Also allow texting such iamges only with new_for_upload() and detect
when that is the case and then run a postprocessing step that converts
that image to a suitable format.
This is done with a new "convert" shader/op.

This now supports all formats natively, no conversions happen on the CPU
anymore (unless the GPU is old).