Depth clamp/clip improvements

[teoxoy]

Based on https://github.com/gpuweb/gpuweb/issues/2100#issuecomment-924536243 & https://github.com/gpuweb/gpuweb/issues/3638#issuecomment-1342646305

DX12

Clamps depth & has DepthClipEnable which corresponds to !unclippedDepth

https://github.com/gfx-rs/wgpu/blob/17143c149c13ea5af36909ef6033e776cecad28c/wgpu-hal/src/dx12/device.rs#L1345

TODO:

• Nothing.

Metal

Has setDepthClipMode available on: iOS 11.0+ & macOS 10.11+

Note: I think if setDepthClipMode is not available, Metal will still clip by default.

if setDepthClipMode is available:

• expose the DEPTH_CLIP_CONTROL feature
• if unclippedDepth { set MTLDepthClipModeClamp } else { set MTLDepthClipModeClip and clamp frag_depth in shader }

else:

• clamp frag_depth in shader

TODO:

• Update the following to check for versions instead and rename the capability to supports_set_depth_clip_mode. https://github.com/gfx-rs/wgpu/blob/17143c149c13ea5af36909ef6033e776cecad28c/wgpu-hal/src/metal/adapter.rs#L737-L739

• Implement frag_depth clamping in the shader

Vulkan

Has depthClamp & VK_EXT_depth_clip_enable

if both are available:

• expose the DEPTH_CLIP_CONTROL feature
• set depthClampEnable
• set depthClipEnable based on unclippedDepth

else if depthClamp is available:

• expose the DEPTH_CLIP_CONTROL feature
• if unclippedDepth { set depthClampEnable } else { clamp frag_depth in shader }

else:

• clamp frag_depth in shader

TODO:

• Shader clamping was implemented in https://github.com/gfx-rs/naga/pull/1431 but wgpu is not setting the flag and it should also clamp between viewport.minDepth and viewport.maxDepth (not 0 and 1)

• Implement the flow above; too many changes to list here (might be useful to undo part of https://github.com/gfx-rs/wgpu/pull/3892)

[JMS55]

I noticed this on a stress test of my renderer (VK backend). When rendering directional light shadow maps with a fragment shader to output unclamped depth, Nvidia NSight says I would get a ~3x speedup in rasterization if ZCULL was enabled, which would require removing the fragment shader and using the pipeline-level depth clamping control.