mesa: Add a helper function for determining the restart index.

The derived state approach currently used (_RestartIndex) doesn't work:
in the GL_PRIMITIVE_RESTART_FIXED_INDEX case, the restart index depends
on the index buffer's data type, and that isn't known until draw time.

The existing code also fails to obey the GL 4.3 rules which say that
FIXED_INDEX takes precedence over normal primitive restart.

This helper function correctly determines the restart index, and will
replace the derived state.

NOTE: This is a candidate for the 9.1 branch.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
(cherry picked from commit 959d076b30)

src/mesa/main/varray.c

@@ -1153,6 +1153,30 @@ _mesa_VertexAttribDivisor(GLuint index, GLuint divisor)
 }
 
 
+unsigned
+_mesa_primitive_restart_index(const struct gl_context *ctx, GLenum ib_type)
+{
+   /* From the OpenGL 4.3 core specification, page 302:
+    * "If both PRIMITIVE_RESTART and PRIMITIVE_RESTART_FIXED_INDEX are
+    *  enabled, the index value determined by PRIMITIVE_RESTART_FIXED_INDEX
+    *  is used."
+    */
+   if (ctx->Array.PrimitiveRestartFixedIndex) {
+      switch (ib_type) {
+      case GL_UNSIGNED_BYTE:
+         return 0xff;
+      case GL_UNSIGNED_SHORT:
+         return 0xffff;
+      case GL_UNSIGNED_INT:
+         return 0xffffffff;
+      default:
+         assert(!"_mesa_primitive_restart_index: Invalid index buffer type.");
+      }
+   }
+
+   return ctx->Array.RestartIndex;
+}
+
 
 /**
  * Copy one client vertex array to another.

src/mesa/main/varray.h

@@ -248,6 +248,8 @@ _mesa_PrimitiveRestartIndex(GLuint index);
 extern void GLAPIENTRY
 _mesa_VertexAttribDivisor(GLuint index, GLuint divisor);
 
+extern unsigned
+_mesa_primitive_restart_index(const struct gl_context *ctx, GLenum ib_type);
 
 extern void
 _mesa_copy_client_array(struct gl_context *ctx,