Use multimap for indexing surfaces (#771)

* Use an std::map for faster texture cache lookup

* Surface cache: Use map instead of multimap, optimize `find` to perform tree scan

* Add comments

include/renderer_gl/surfaces.hpp

@@ -172,4 +172,4 @@ struct DepthBuffer {
 	size_t sizeInBytes() {
 		return (size_t)size.x() * (size_t)size.y() * PICA::sizePerPixel(format);
 	}
-};
+};