gfx: Abort loading data file if it is compressed
map: Deallocate map data before loading new map

Also deallocate everything on program exit.
viewport: Properly deallocate previous landscape tiles when reinitialized
viewport: Allow left mouse button to be used for dragging

This is needed for touchpads that do not have physical secondary mouse button.
player: Fix bug where uninitialized timers_count causes crash
sdl-video: Work around SDL bugs in AmigaOS