This fixes a number of things that are warnings in the Mac compiler....

core/cross/gl/utils_gl.cc

Index: core/cross/gl/utils_gl.cc
===================================================================
--- core/cross/gl/utils_gl.cc	(revision 21208)
+++ core/cross/gl/utils_gl.cc	(working copy)
@@ -194,7 +194,7 @@
   //
   // TODO: make this a runtime provided value discovered during
   // Renderer creation.
-  const int kMaxAttrIndex = 15;
+  const unsigned int kMaxAttrIndex = 15u;
   if (attr > kMaxAttrIndex) {
     //TODO: Figure out how to get errors out of here to the client.
     DLOG(ERROR) << "Invalid vertex attribute index.";