Texture Format Swizzling

[TheSpydog]

Some of the texture formats we expose are only natively supported by Vulkan, but they have swizzled variants that are supported by all the other APIs. I think we should expose the most common formats and have fallback swizzling as needed for the backends that lack native support.

• R5G6B5: Replace with B5G6R5 for D3D and Metal. Vulkan has 69.62% coverage for B5G6R5 as a native sampler format.
• A1R5G5B5: Replace with B5G5R5A1 for D3D and Metal. Vulkan has 83.39% coverage for BGG5R5A1 as a native sampler format.
• B4G4R4A4: Supported by Vulkan and D3D, but not by Metal. Since Metal's the odd one out, we can swizzle its A4B4G4R4 format.
• A2R10G10B10: Replace with R10G10B10A2 for D3D and Metal. Vulkan has no native support for that swizzling.
• A2B10G10R10: Replace with B10G10R10A2 for D3D and Metal. Vulkan has no native support for that swizzling.

[flibitijibibo]

Out of all the APIs Vulkan has the best support for built-in swizzling, we should use the formats that other APIs natively support and swizzle as needed for Vulkan (or use a native format if an extension like EXT_4444_formats is available). FNA has been able to handle this, so we can take notes from our own Vulkan renderer if needed.