Metal: Fixed texture creation.

src/renderer_mtl.mm

@@ -2916,7 +2916,7 @@ namespace bgfx { namespace mtl
 				temp = (uint8_t*)BX_ALLOC(g_allocator, ti.width*ti.height*4);
 			}
 
-			for (uint8_t side = 0; side < numSides; ++side)
+			for (uint16_t side = 0; side < numSides; ++side)
 			{
 				uint32_t width  = ti.width;
 				uint32_t height = ti.height;