document pt_image_tile_* as semi-threadsafe, and nogil pypngtile.pyx (and get rid of trap_err, ugh). Also fix constness warning

syntax: regexp

\.[^/]+\.sw[op]$
\.pyc$

^build/
^bin/pngtile$
^lib/libpngtile.so$
^doc/html
^dist
^Learning Diary\.pdf$