Maintain the bounding box of drawables (i.e., the bounds of their
actual rendered content) separately from their logical boundary (as
shown in the UI).

The bounding box is calculated through the new
GimpDrawable::get_bounding_box() virtual function, which has a
corresponding gimp_drawable_get_bounding_box() function; the
default implementation simply returns the drawable's logical
boundary.  The bounding box is specified in drawable coordinates,
i.e., it's not affected by the drawable's offset.

The bounding box is recalculated through
gimp_drawable_update_bounding_box(), which should be called
whenever a change may affect the bounding box (for example, when
setting a new buffer, as done implicitly by GimpDrawable's
::set_buffer() implementation, or when a drawable filter's
properties change, as will be done by GimpDrawableFilter in a
following commit).  When the bounding box changes, the affected
regions of the drawable are updated...