SciTools/cartopy — pull request #416 by rhattersley (2014-05-14 09:19:36+00:00)
Title: Clear image accumulator to avoid ghosting.

Currently the `big_img` isn't cleared before use, so if some of the sources tiles don't exist you can end up with uninitialised data in the final image.

This PR clears `big_img` to fully transparent before use.