Fixed missing transparency on upscaled textures

Textures with diagonal patterns were treated as opaque after resizing
Images upscaled by hqNx were affected mostly by this issue

http://forum.drdteam.org/viewtopic.php?f=24&t=5370
http://zandronum.com/tracker/view.php?id=269
http://zandronum.com/tracker/view.php?id=315