gl_VertexID implementation

This cl implements support for gl_VertexID.

Passes the functional.shaders.builtin_variable.vertex_id test.

Change-Id: I5550e3ecba30e29f1e38ace608d730833a1e9598
Reviewed-on: https://swiftshader-review.googlesource.com/10958
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/src/Shader/PixelProgram.cpp b/src/Shader/PixelProgram.cpp
index ecf164f..948f103 100644
--- a/src/Shader/PixelProgram.cpp
+++ b/src/Shader/PixelProgram.cpp
@@ -846,8 +846,8 @@
 			reg = v[2 + i];
 			break;
 		case Shader::PARAMETER_MISCTYPE:
-			if(src.index == 0) reg = vPos;
-			if(src.index == 1) reg = vFace;
+			if(src.index == Shader::VPosIndex) reg = vPos;
+			if(src.index == Shader::VFaceIndex) reg = vFace;
 			break;
 		case Shader::PARAMETER_SAMPLER:
 			if(src.rel.type == Shader::PARAMETER_VOID)