Expose BGRA images
Gallium uses BGRA storage images for PBO ReadPixels(). Iris does not support these - iris_is_format_supported returns false because BGRA formats don't support typed reads. We should support them by internally using RGBA and swizzling, possibly in the shader, maybe updating nir_lower_image_load_store.
GLSL doesn't allow BGRA images in the source language today, so this is a little awkward - there's no image qualifier for it. We may need to make one internally. @jekstrand was also thinking of making a SPIR-V extension to allow them in Vulkan.
If we support that, we'll get PBO downloads.