mesa: Make gl_VertexID be a system value like gl_InstanceID.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/builtin_variables.cpp

@@ -98,7 +98,7 @@ static const builtin_variable builtin_120_fs_variables[] = {
 };
 
 static const builtin_variable builtin_130_vs_variables[] = {
-   { ir_var_in,  -1,                 "int",   "gl_VertexID" },
+   { ir_var_system_value,  SYSTEM_VALUE_VERTEX_ID, "int",   "gl_VertexID" },
 };
 
 static const builtin_variable builtin_110_deprecated_uniforms[] = {

src/mesa/main/mtypes.h

@@ -1798,6 +1798,7 @@ typedef enum
 typedef enum
 {
    SYSTEM_VALUE_FRONT_FACE,  /**< Fragment shader only (not done yet) */
+   SYSTEM_VALUE_VERTEX_ID,   /**< Vertex shader only */
    SYSTEM_VALUE_INSTANCE_ID, /**< Vertex shader only */
    SYSTEM_VALUE_MAX          /**< Number of values */
 } gl_system_value;