Red Alert building artwork polishing

This fixes #2660 and adds [Red Alert Building Husks](http://www.sleipnirstuff.com/forum/viewtopic.php?f=83&t=16100) made by Harisson and other unpublished artwork he labeled as *[OpenRa]Red Alert eX.png* in the e-Mail he sent me:
* [smoother weapon factory Door animation](http://content.open-ra.org/?p=detail&table=units&id=306)
* [more ore silo fill steps](http://content.open-ra.org/?p=detail&table=units&id=305)