Make levelshot command take 2 pictures like used in Team Arena

screenshot* levelshot now saves images at screen resolution to
levelshots/mapname.ext, and a resized image to mapname_small.ext.

The size to resize can be set by "screenshot* levelshot <w> <h>",
height defaults to width.
Add support for levelshots/*_small to q3_ui

Remove extention from levelshot/* in cgame.