map preview generation reworked like nemo suggested (he was right, it's quickier in this way) add some rounding image capabilities for more professional look

Engine: * Added texture priorities to try to keep often used sprite textures as well as huge textures in video memory - this should improve performance on systems with low video memory * Fixed portal gun's weapon offset

a bunch of minor stuff

Engine: * Removed horizontal weapon offsets for now (shouldn't affect human player's gameplay but that of AI) Frontend: * Added some main menu tips