Make DX12 the Default API on Windows

[cwfitzgerald]

For a variety of reasons, we want to make DX12 the default API on windows:

• DXGI swapchains work a lot better on windows than vulkan WSI, particuarly AMD and Intel's. They are much more stable and more well behaved.
• Cleaner interop with native apis.
• Better debug layer and ease of bug reports as well as access to tools like PIX.

Before we make the switch, we need to get the DX12 backend tested and the major holdups fixed. This isn't a complete list and please add things as they are found.

Blocking Issues:

• Suballocate Buffers in DX12 #2720
• Creating a Window with DX12 and Immediate Mode Causes Error in ResizeBuffers #2712
• DXC Integration in DX12 Backend #2722
• Implement Push Constants for DX12 #2829
• Audit D3D12 Features and Limits wrt Resource Binding Tiers #3333
• DX12 Sampler Heap Requirements Overly Strict #3350
• [hlsl-out] Arrays in Structs Are Incorrectly Loaded When Loaded as A Whole Struct naga#2184
• [hlsl-out] Stores of vec2-based matrix inside struct tries to access raw matrix directly #4423
• dx12 panic with shared shader file using push constants #3624
• Crash on queue_submit #3617
• AMD on DX12 Renders Nothing on msaa-line Example #3838
• Resize error on DX12 backend in wgpu 0.17 #3967
• Investigate spirv_to_dxil Library #4155
• Enable DX12 GPU Based Validation #4291
• [D3D12] Fix vertex_index and instance_index built-in inputs for indirect draw calls #2471

There are some driver issues that we may not be able to fix, but we should keep in mind:

• WARP: Uniform Values Arriving As Zero #2296
• Queue::write_buffer() issue on Microsoft Basic Render Driver #2189

[ErichDonGubler]

We're interested in this in the Firefox org. Here's our own Bugzilla entry tracking this: bug 1836816

[ErichDonGubler]

@nical has made Firefox's usage of wgpu-core default to DX12 as the only backend on Windows: bug 1853140

This should indirectly help this issue, since fixing the most salient DX12 issues is our priority, for the time being.