Provide explicit access to SDL 3's Direct3D 12 GPU backend

[nmlgc]

SDL 3's GPU renderer backend is implemented for Vulkan, Direct3D 12, and Metal, with Vulkan being the default outside of macOS and iOS. This forms a second layer of backends, and the current API menu only covers the first one. As such, you currently can't override this default:

Probably very academic and minor, but SDL 3 is highly likely to throw out the non-GPU Direct3D 12 renderer backend because there is seemingly no more need for it now that the GPU implementation exists. (Also, fewer backends = smaller code surface = fewer bugs, I'm fully with them in that regard.) This feature will probably be the only way users could continue to choose Direct3D 12 going forward.