MSL: Expand quad gl_TessCoord to a float3.

This is the actual SPIR-V type of the builtin. We forced to a `float2`
in the declaration because that's what Metal wants.

reference/opt/shaders-msl/tese/water_tess.tese

@@ -29,9 +29,9 @@ struct main0_patchIn
 [[ patch(quad, 0) ]] vertex main0_out main0(main0_patchIn patchIn [[stage_in]], constant UBO& _31 [[buffer(1)]], texture2d<float> uHeightmapDisplacement [[texture(0)]], sampler uHeightmapDisplacementSmplr [[sampler(0)]], float2 gl_TessCoord [[position_in_patch]])
 {
     main0_out out = {};
-    float2 _201 = patchIn.vOutPatchPosBase + (gl_TessCoord.xy * _31.uPatchSize);
-    float2 _214 = mix(patchIn.vPatchLods.yx, patchIn.vPatchLods.zw, float2(gl_TessCoord.x));
-    float _221 = mix(_214.x, _214.y, gl_TessCoord.y);
+    float2 _201 = patchIn.vOutPatchPosBase + (float3(gl_TessCoord, 0).xy * _31.uPatchSize);
+    float2 _214 = mix(patchIn.vPatchLods.yx, patchIn.vPatchLods.zw, float2(float3(gl_TessCoord, 0).x));
+    float _221 = mix(_214.x, _214.y, float3(gl_TessCoord, 0).y);
     float _223 = floor(_221);
     float2 _125 = _201 * _31.uInvHeightmapSize;
     float2 _141 = _31.uInvHeightmapSize * exp2(_223);

reference/shaders-msl/tese/water_tess.tese

@@ -50,7 +50,7 @@ float3 sample_height_displacement(thread const float2& uv, thread const float2&
 [[ patch(quad, 0) ]] vertex main0_out main0(main0_patchIn patchIn [[stage_in]], constant UBO& v_31 [[buffer(1)]], texture2d<float> uHeightmapDisplacement [[texture(0)]], sampler uHeightmapDisplacementSmplr [[sampler(0)]], float2 gl_TessCoord [[position_in_patch]])
 {
     main0_out out = {};
-    float2 tess_coord = gl_TessCoord.xy;
+    float2 tess_coord = float3(gl_TessCoord, 0).xy;
     float2 param = tess_coord;
     float2 pos = lerp_vertex(param, patchIn.vOutPatchPosBase, v_31);
     float2 param_1 = tess_coord;

spirv_msl.cpp

@@ -7147,6 +7147,14 @@ void CompilerMSL::bitcast_from_builtin_load(uint32_t source_id, std::string &exp
 
 	if (expected_type != expr_type.basetype)
 		expr = bitcast_expression(expr_type, expected_type, expr);
+
+	if (builtin == BuiltInTessCoord && get_entry_point().flags.get(ExecutionModeQuads) && expr_type.vecsize == 3)
+	{
+		// In SPIR-V, this is always a vec3, even for quads. In Metal, though, it's a float2 for quads.
+		// The code is expecting a float3, so we need to widen this.
+		expr = join("float3(", expr, ", 0)");
+	}
+
 }
 
 void CompilerMSL::bitcast_to_builtin_store(uint32_t target_id, std::string &expr, const SPIRType &expr_type)