can't be synchronous on Android: you can't block the main UI thread.
So now there's a new background thread to which all jni activity
except game opening and saving is delegated (and these happen before
and after the thread exists.) Tile picking works because the request
comes in on the background thread which then blocks until the
BoardActivity reports that the tile picker's gone away.
canvas and blit that to draw's canvas each time. Requires keeping x
and y offset since board doesn't fill the view; requires erasing tiles
and score entries before drawing.
