DX12 Test Build 1

This is an initial pre-release build of the new DirectX 12 rendering backend.

Current issues:

• Only 5 million notes can be rendered at once
• FFmpeg mode doesn't work
• Non-ASCII markers don't render properly
• GPU can't be manually selected
• Probably some more issues I forgot

HOTFIX 1: Reduce D3D12 support requirements
HOTFIX 2: Fix upload buffer flags
HOTFIX 3: Fix Intel iGPU support