You can now set any paint tool to mirror painting relatively
horizontal/vertical axis or a central point (any combination of these 3
symmetries).
This has been implemented as a new multi-stroke core, where every stroke
is actually handled as a multi-stroke (default of size 1).
This is also the first usage of custom guides for symmetry guiding.
Current version has to be activated in the playground.
Change gimp_pixbuf_create_buffer() to copy the pixels if a linear
buffer cannot be created. Add functions that convert between
GimpTempBuf and GdkPixbuf. Fix users of gimp_pixbuf_create_buffer()
to make the least possible copies. Patch modified by Mitch.
