ZENGINE: renderImageToScreen: Check for TGZ first instead of TGA

TGA's aren't required to have a magic number in the header, but TGZ are.
Therefore it's easier to identify TGZ files.
ZENGINE: Move engine width, height, and pixelFormat to const member variables
ZENGINE: Create/refactor methods for playing video.

The pixel format for videos is not the same as for the rest of the game.
(Game: RGB 555, Video: RGB 565)
ZENGINE: Create console command for loading a video