In gimpchannel-select, move some of the common functionality of the
various gimp_channel_select_foo() functions to gimpchannel-combine.
Furthermore, don't special-case CHANNEL_OP_INTERSECT, but rather
pass it over to gimpchannel-combine, which is now prepared to
handle it in all functions, as per the previous commits.

In gimpchannel-combine, factor out the common functionality of the
various gimp_channel_combine_foo() functions into a pair of
gimp_channel_combine_{start,end}() functions, which are called
before/after the actual gimp_gegl_mask_combine_foo() function,
respectively.  In particular, these functions deal with calculating
the new channel bounds.  Previously, the various
gimp_gegl_mask_combine_foo() functions would implicitly invalidate
the channel bounds (since commit
d0ae244f), rendering the bounds-
recalculation code ineffective.  This avoids manually recalculating
the bounds in many cases, speeding up selection operations.