In GimpDrawable::set_buffer(), and the corresponding
gimp_drawable_set_buffer_full() function, take a bounds rectangle,
which specifies both the drawable's new offset and its new size,
instead of only taking the new offset.  In
gimp_drawable_real_set_buffer(), set the item size according to the
rect dimensions, instead of the buffer dimensions.  The rect's
width/height may be 0, in which case the buffer's dimensions are
used.

Adapt the rest of the code.

We do this in preparation for maintaining the drawable's bounding
box separately from its logical bounds, allowing the drawable
content to extend beyond its bounds.