Support PNG alpha channels

[jacobhutson]

please please please fix support for PNG transparency maps.
PNG + Alpha is a standard format for entourage, leaf maps etc

[3DsChobo]

yes plz. this is quite annoying, though there is a way around it.

- use the same png that contains the rgb in the opacity slot
- use the alpha as grey radiobutton
- insert a color correction map and invert the alpha (the invert button on the output section wont work)

[Muadib]

yeah i mostly use some colour correction workaround but having this option would be really good.

[Ondra]

Fixed in 1.2