[vk] Check for BGRA8 linear image to support blitting

[kvark]

The result of upload_image_simple is used as a blit source, so we need to make sure it's possible.

[kd-11]

Return 'nullptr' for consistency. Also add a space between // and comment body. It was decided as the preferred method for new code and also makes clang-format happy

[kvark]

@kd-11 thanks for the review! comments are addressed now.
Also renamed the variable to just bgra8_linear since the other ones also check for blit support and the don't reflect that with _blit suffix.

[kd-11]

Its ok now. Its quite strange to have a GPU with the BGRA8 format not usable as a blit source, I wonder how many other formats cannot be easily copied to the backbuffer via blitting.

[kvark]

@kd-11
Copying is fine, it's the blitting that's not so easy. Blitting is different in that it requires a graphics queue, and can be implemented under the hood as a graphics operation.

Also, it's not BGRA8 that is problematic but the fact of placing it in HOST_VISIBLE memory (with linear tiling). Vulkan baseline requirements for HOST_VISIBLE images are super low, e.g. only 2D with a few formats, and they aren't required to support any graphics operations. Basically, they are glorified staging buffers, and that's how Metal portability backend implements them. Hence, they can only be used in transfers.

[kd-11]

Also, it's not BGRA8 that is problematic but the fact of placing it in HOST_VISIBLE memory.

Makes sense, otherwise it would have been strange as drivers often just implement blitting with a full-quad pass which would have made the user-side implementation of the same redundant. Thanks for that clearing up.