Determine which dawn backends to enable automatically

[zeozeozeo]

I wasn't able to build the D3D11/D3D12 backends on mingw so they are disabled on mingw for now:

[91/252] Building CXX object _deps/dawn-build/src/dawn/native/CMakeFiles/dawn_native_objects.dir/Instance.cpp.obj
FAILED: [code=1] _deps/dawn-build/src/dawn/native/CMakeFiles/dawn_native_objects.dir/Instance.cpp.obj
C:\PROGRA~1\mingw64\bin\CLANG_~1.EXE -DDAWN_ENABLE_ASSERTS -DDAWN_ENABLE_BACKEND_D3D11 -DDAWN_ENABLE_BACKEND_D3D12 -DDAWN_ENABLE_BACKEND_NULL -DDAWN_ENABLE_BACKEND_VULKAN -DNOMINMAX -DTINT_BUILD_GLSL_VALIDATOR=0 -DTINT_BUILD_GLSL_WRITER=0 -DTINT_BUILD_HLSL_WRITER=0 -DTINT_BUILD_IR_BINARY=0 -DTINT_BUILD_IS_LINUX=0 -DTINT_BUILD_IS_MAC=0 -DTINT_BUILD_IS_WIN=1 -DTINT_BUILD_MSL_WRITER=0 -DTINT_BUILD_SPV_READER=0 -DTINT_BUILD_SPV_WRITER=1 -DTINT_BUILD_TINTD=0 -DTINT_BUILD_WGSL_READER=1 -DTINT_BUILD_WGSL_WRITER=1 -DTINT_ENABLE_IR_VALIDATION=1 -DWIN32_LEAN_AND_MEAN -isystem C:/Code/probe/build/_deps/dawn-src/include -isystem C:/Code/probe/build/_deps/dawn-build/gen/include -isystem C:/Code/probe/build/_deps/dawn-src/src/dawn/partition_alloc -isystem C:/Code/probe/build/_deps/dawn-src/third_party/vulkan-headers/src/include -isystem C:/Code/probe/build/_deps/dawn-src/third_party/vulkan-utility-libraries/src/include -isystem C:/Code/probe/build/_deps/dawn-src/third_party/abseil-cpp -isystem C:/Code/probe/build/_deps/dawn-src -isystem C:/Code/probe/build/_deps/dawn-src/src -isystem C:/Code/probe/build/_deps/dawn-build/gen/src -fPIC -g -fno-rtti -std=c++20 -Wno-c++20-compat -fno-exceptions -fno-rtti -Wno-assume -Wno-deprecated-builtins -Wno-unknown-warning-option -Wno-switch-default -Wno-nrvo -Wno-gcc-compat -Wno-unreachable-code-break -Wno-nullability-extension -Wno-shadow -MD -MT _deps/dawn-build/src/dawn/native/CMakeFiles/dawn_native_objects.dir/Instance.cpp.obj -MF _deps\dawn-build\src\dawn\native\CMakeFiles\dawn_native_objects.dir\Instance.cpp.obj.d -o _deps/dawn-build/src/dawn/native/CMakeFiles/dawn_native_objects.dir/Instance.cpp.obj -c C:/Code/probe/build/_deps/dawn-src/src/dawn/native/Instance.cpp
In file included from C:/Code/probe/build/_deps/dawn-src/src/dawn/native/Instance.cpp:57:
In file included from C:/Code/probe/build/_deps/dawn-src/src/dawn/native/d3d/BackendD3D.h:39:
In file included from C:/Code/probe/build/_deps/dawn-src/src/dawn/native/d3d12/d3d12_platform.h:31:
C:/Code/probe/build/_deps/dawn-src/src/dawn/native/d3d/d3d_platform.h:44:10: fatal error: 'DXProgrammableCapture.h' file not found
   44 | #include <DXProgrammableCapture.h>  // NOLINT(build/include_order)
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.

[Peter0x44]

Sorry for the delay reviewing this.
Now that 97d79ea has possibly made GL work, should we be enabling it here too?

[Peter0x44]

Does the dawn NULL backend need to be on for this case?

-- Dawn build D3D11 backend: OFF
-- Dawn build D3D12 backend: OFF
-- Dawn build Metal backend: OFF
-- Dawn build Vulkan backend: OFF
-- Dawn build OpenGL backend: OFF
-- Dawn build OpenGL ES backend: OFF
-- Dawn build Null backend: OFF

Currently a dawn build without SUPPORT_WGPU_BACKEND (but with SUPPORT_WGSL_PARSER) doesn't enable the null backend.