gpu: Improve SupportsShaderFormat runtime check

[flibitijibibo]

At the moment our format check is basically just a static bitflag comparison and the formats are hardcoded into each bootstrap. This is okay but it's not ideal for a few reasons:

• When Shader Model 7 comes out for D3D12 we can't just assume SPIR-V is available, we have to check the feature level first
• On macOS Metal is technically always available but the device family may not meet SDL's needs
• At least one NDA platform only supports a specific shader format when an on-disk shader cache is present

So we need to beef this up a bit and make it a whole bootstrap function, where drivers are given a chance to check for minimum support in addition to checking if the backend recognizes a specific format. For example, we should move this check from CreateDevice to CheckSupport:

https://github.com/libsdl-org/SDL/blob/main/src/gpu/metal/SDL_gpu_metal.m#L4472-L4488

[flibitijibibo]

As a band-aid to the Mac problem we can maybe check for basic hardware support in PrepareDriver, but that's not quite the same as driver format support.