Black squares in some textures

[Graf Zahl]

Too bad that some people need those shitty filters. They've been a pain in the ass since they were forced upon me.

[_mental_]

Can be fixed like this. Honestly this function needs to be refactored but I would like to fix the bug first.
Also, if masked flags is cleared in FTexture::ProcessData() function because of usage of some filter, it will be never set again. Maybe there is a reason for this though.