1. Add ALLOW_RENDER_TARGET flag for color textures in CreateTexture
- This was the root cause of 5 failing RTV-related tests
- Without this flag, creating RTV caused device removal
2. Add FromD3D12() reverse conversion for Format enum
- GetFormat() was incorrectly casting DXGI_FORMAT to Format
- DXGI_FORMAT_R8G8B8A8_UNORM=28 but Format::R8G8B8A8_UNorm=3
- Added FromD3D12() to properly convert back
3. Update RHITestFixture to pre-create CommandQueue and Fence
- Prevents potential timing issues with GPU synchronization
4. Update RHITestFixture tests to pass correct format in ResourceViewDesc
- Previously passed empty desc.format=0 which caused issues
All 234 RHI unit tests now pass (117 D3D12 + 117 OpenGL)
- Add RHITestFixture with RHI_BACKEND env var support for backend selection
- Add unit tests for: Device, Buffer, Texture, SwapChain, CommandList, CommandQueue, Shader, Fence, Sampler
- Tests can run against D3D12 or OpenGL backends via RHI_BACKEND env var
- Add integration folder placeholder for future integration tests
