Make the Surface.get_at_mapped method value consistent with Surface.map_rgb
Surface.map_rgb returns a signed Python int, which can be negative for some pixel values. Surface.get_at_mapped returned strictly positive pixel values. This lead to a unit test failure in pixelcopy_test.py for an x86_64 Pygame build, though the test should have passed. Since get_at_mapped was added for testing purposed, its return value should be consistent with map_rgb, which predates get_at_mapped.