Transparency in markers is not handled correctly

[sapiezynski]

while loading a png marker pyglet seems to be blending the transparent pixels with black instead of with whatever there is below them. Therefore, for example multiple red half-transparent markers on top of one another become black instead of red.

Possible fix: http://stackoverflow.com/questions/27827547/load-image-with-pyglet-turns-some-transparency-pixels-white https://gist.github.com/jamesadney/1975776