Flip the image in read_pixels and test read_pixels

- In read_pixels, the result image is upside-down. Flip the image using `[::-1]`.
- Unit test for read_pixels: Draw a white square on top left, check sums of pixels in corners to compare result.

This should just be `from vispy.gloo import read_pixels`, so you'll need to add `read_pixels` to `vispy/gloo/__init__.py` if it's not already there. (Flatter namespace is better than nested)