Add an option to rescale graphics in a WAD file, perhaps several at once

[flarn2006]

SLADE Version: 3.1.11 Operating System: Linux

Issue Details: Several times, I've imported images into SLADE, only to realize I forgot to scale them up. But for some reason, SLADE doesn't seem to have any way to do this, other than exporting the images, rescaling them in another program, and reimporting them. It would be very convenient to have a way to do this in SLADE.

I think this is especially an issue for Sonic Robo Blast 2, as that game is at a different scale; I'm pretty sure one fracunit in Doom is twice as long as a fracunit in SRB2.