render: Added Xbox DXIL for gpu backend

[flibitijibibo]

This gets Xbox running with render_gpu, but I got the root signatures all wrong - will need someone more familiar with D3D12 to get working.

[Starkium]

this is awesome, but does it matter anymore? xbox is kind of dead. Might be good for the modding / homebrew scene if they can unlock the Xboxes to use more than UWP. Maybe that's already the case, I'm a few years behind.

[TheSpydog]

Sorry, it looks like this broke the GDK build, so we should revert this change. Xbox precompilation fails without root signature definitions, but the good news is that the original dxil.h just works, so we don't actually need any changes to what's already there for Windows!

[flibitijibibo]

Sure, we can revert this... how does the PC DXIL work though? Is this something cert would complain about?

[TheSpydog]

Nope, it should be totally fine. DXIL isn't directly consumed, it's just an intermediate format. Xbox can compile shaders at runtime from both HLSL and DXIL source (which is how ShaderCross and GLon12 work). It's just not as optimal as full precompilation, which allows the Xbox to skip the DXIL -> machine code step at runtime.

[flibitijibibo]

This has been reverted in main and release-3.4.x.