MSL: Adjust FragCoord for sample-rate shading.

In Metal, the `[[position]]` input to a fragment shader remains at
fragment center, even at sample rate, like OpenGL and Direct3D. In
Vulkan, however, when the fragment shader runs at sample rate, the
`FragCoord` builtin moves to the sample position in the framebuffer,
instead of the fragment center. To account for this difference, adjust
the `FragCoord`, if present, by the sample position. The -0.5 offset is
because the fragment center is at (0.5, 0.5).

Also, add an option to force sample-rate shading in a fragment shader.
Since Metal has no explicit control for this, this is done by adding a
dummy `[[sample_id]]` which is otherwise unused, if none is already
present. This is intended to be used from e.g. MoltenVK when a
pipeline's `minSampleShading` value is nonzero.

Instead of checking if any `Input` variables have `Sample`
interpolation, I've elected to check that the `SampleRateShading`
capability is present. Since `SampleId`, `SamplePosition`, and the
`Sample` interpolation decoration require this cap, this should be
equivalent for any valid SPIR-V module. If this isn't acceptable, let me
know.

1 files changed, 4 insertions, 0 deletions

diff --git a/spirv_cross_c.cpp b/spirv_cross_c.cpp
index 4b561378..d90d40ab 100644
--- a/spirv_cross_c.cpp
+++ b/spirv_cross_c.cpp
@@ -690,6 +690,10 @@ spvc_result spvc_compiler_options_set_uint(spvc_compiler_options options, spvc_c
 	case SPVC_COMPILER_OPTION_MSL_FIXED_SUBGROUP_SIZE:
 		options->msl.fixed_subgroup_size = value;
 		break;
+
+	case SPVC_COMPILER_OPTION_MSL_FORCE_SAMPLE_RATE_SHADING:
+		options->msl.force_sample_rate_shading = value != 0;
+		break;
 #endif
 
 	default: