Add support for PPM graphics to SyncTERM

You can copy whole, or portions of a PPM file from the cache directory
to the screen, optionally applying a mask.  The mask can be specified
as a base64-encoded bitmap or as a PBM file in the cache directory.

You can also copy portions of the screen into an internal buffer, and
draw from that internal buffer.  There's plans to support loading an
image file from the cache directory into the buffer as well.

Portions of the buffer can be copied to the screen, optionally
applying a mask as with drawing PPMs.

There's also plans to support copying a PBM file into a mask buffer.

I've added the syncterm-bounce.js script which simply bounces the
SyncTERM icon around the screen like the classic screensaver we all
hate.  I initially wanted to replace the spinning cursor with it,
but there doesn't seem to be a way to replace that with a script. :(

To use the script, just add the text:
@EXEC:syncterm-bounce.js@ to the end of your logon.js script.

Log in to nix.synchro.net with the latest nightly build or build from
git to see the script in action.