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

core/cross/bitmap.cc

 /*
  * Copyright 2009, Google Inc.
  * All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 597 matching lines @@
       DCHECK(y_contrib <= dst_height);
       unsigned int contrib = x_contrib * y_contrib;
       for (unsigned int c = 0; c < components; ++c) {
         accum[c] +=
             contrib * src_data[(src_y * src_width + src_x) * components + c];
       }
     }
   }
   for (unsigned int c = 0; c < components; ++c) {
     uint64 value = accum[c] / (src_height * src_width);
-    DCHECK_LE(value, 255);
+    DCHECK_LE(value, 255u);
     dst_data[(y * dst_width + x) * components + c] =
         static_cast<unsigned char>(value);
   }
 }
 
 bool Bitmap::GenerateMipmaps(unsigned int base_width,
                              unsigned int base_height,
                              Texture::Format format,
                              unsigned int num_mipmaps,
                              unsigned char *data) {
@@ skipping 337 matching lines @@
       return false;
   }
   return true;
 }
 
 ObjectBase::Ref Bitmap::Create(ServiceLocator* service_locator) {
   return ObjectBase::Ref(new Bitmap(service_locator));
 }
 
 }  // namespace o3d