vulkan: Combine textures and samplers again

vulkan: Combine textures and samplers again

This reverts most of commit f420c143e0d8947a433704969bab02aea47d82f5
again because it turns out GPUs like combined images and samplers.

But: The one thing we don't revert is allowing the C code to select any
combination of sampler and image:
gsk_vulkan_render_get_image_descriptor() now takes a 2nd argument
specifying the sampler.

This allows the same flexibility as before, we just combine things
early.

This change was inspired by
https://developer.nvidia.com/blog/vulkan-dos-donts/