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

core/cross/gl/texture_gl.h

Index: core/cross/gl/texture_gl.h
===================================================================
--- core/cross/gl/texture_gl.h	(revision 21208)
+++ core/cross/gl/texture_gl.h	(working copy)
@@ -119,7 +119,7 @@
 
   // Returns true if the backing bitmap has the data for the level.
   bool HasLevel(unsigned int level) const {
-    DCHECK_LT(level, levels());
+    DCHECK_LT(static_cast<int>(level), levels());
     return (has_levels_ & (1 << level)) != 0;
   }
 
@@ -196,7 +196,7 @@
 
   // Returns true if the backing bitmap has the data for the level.
   bool HasLevel(unsigned int level, CubeFace face) const {
-    DCHECK_LT(level, levels());
+    DCHECK_LT(static_cast<int>(level), levels());
     return (has_levels_[face] & (1 << level)) != 0;
   }