map.c: write_image(): fix portability problem

When built with Qt and saving in RGB format, we just copied out the
archictecture-dependent qRgb.  Now unpack the int into pixels and
write that out.
map.c: text corrections
map.c: make it a nanosecond faster when writing RGB image with Qt

By using bit operations.
cleanup

  * move tasw-examples to hildon-specific section
  * indentation fixes
  * error message capitalization fixes
  * code commentary enhancements
add get_size() and get_geometry_nok()

  * get_geometry() calls get_geometry_nok(), which does the real work
  * get_geometry_nok() calls get_size(), which is also used to obtain
    a <size>
  * use get_size() in -G dflt=... because that's how it's specified
implement image plotting

  * -G img=,,, and -o bg=...
  * can use gdk-pixbuf Qt to load images and/or Xrender to scale it
  * add alpha_mask() as a separate function