HLSL -> MSL: raster_order_group annotation

[jesusdz]

Hi there!

We're trying to add support for raster order groups while translating from HLSL to MSL, which is currently missing in SPIRV-Cross apparently.

We compilie HLSL -> SPIRV, then use SPIRV-Cross to generate SPIRV -> MSL, so ideally, we think it would be convenient to annotate HLSL in a way similar to this:

// 5635 is the number for DecorationUserSemantic / DecorationHlslSemanticGOOGLE
struct GBuffer
{
	[[vk::location(0), vk::ext_decorate_string(5635, "raster_order_group(0)")]]	float4 rt0 : SV_TARGET0;
	[[vk::location(1), vk::ext_decorate_string(5635, "raster_order_group(0)")]]	float4 rt1 : SV_TARGET1;
	[[vk::location(2), vk::ext_decorate_string(5635, "raster_order_group(0)")]]	float4 rt2 : SV_TARGET2;
	[[vk::location(3), vk::ext_decorate_string(5635, "raster_order_group(1)")]]	float4 rt3 : SV_TARGET3;
};

so an OpMemberDecorate with the user semantic is embedded into the SPIRV. Having this, we could introduce some changes into SPIRV-Cross to finally produce some MSL like this:

struct ps_main_out
{
    float4 out_var_SV_TARGET0 [[color(0), raster_order_group(0)]];
    float4 out_var_SV_TARGET1 [[color(1), raster_order_group(0)]];
    float4 out_var_SV_TARGET2 [[color(2), raster_order_group(0)]];
    float4 out_var_SV_TARGET3 [[color(3), raster_order_group(1)]];
};

I am not sure this is the better approach to achieve this. Is it possible there's a better approach rather than introducing this kind of hacky annotation in the SPIRV code?

Cheers!