Merge pull request #69 from groessler/something_to_pull

Mouse drivers for Atari
Trakball mouse driver
Detect pen button. If we read 228 for X or Y position, don't change cursor
position, since the pen was probably lifted from the pad.
some comment fixes
fix bug where the high byte of X and Y positions were destroyed of
the user had lifted the pen
Always print the mouse cursor, even if coordinates haven't changed.
This makes sure that the cursor is always visible, even if the program
has written text to the screen (only valid for non-P/M mouse callbacks).
Adapt to new mouse driver interface ('prep' and 'draw')