MSL: Add reversed depth viewport emulation#2640
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Add an MSL option to emulate reversed depth viewports by inverting clip-space Z in vertex-like shaders.
The option binds a per-viewport mask and applies:
position.z = position.w - position.z
when the selected viewport bit is set. If the shader writes ViewportIndex, use that output to select the mask bit; otherwise use viewport 0.
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This add an opt-in MSL backend option to emulate reversed depth viewports by inverting clip-space Z in vertex-like shaders.
The option originated from the MoltenVK workaround discussion in KhronosGroup/MoltenVK#2760
On some affected Metal implementations, passing a reversed depth viewport directly to Metal can fail. The intended consumer is MoltenVK, which can normalize the Metal viewport depth range and ask SPIRV-Cross to emulate the missing reversed-depth transform in generated MSL
When enabled, SPIRV-Cross adds a
constant uint&mask argument and emits:for viewports whose mask bit is set.
Changes: