Override GimpLayer::get_effective_mode() in GimpGroupLayer, to
perform strength-reduction of pass-through groups to normal groups
under certain conditions (see gimp_group_layer_get_effective_mode()
for the logic.)

The main motivation for this is the fact that Photoshop uses pass-
through mode as the default mode for groups, resulting in many PSDs
using pass-through groups generously and unnecessarily.  Since
pass-through groups are more expensive that normal groups, reducing
them to normal groups when possible can make a big difference.

Note that, while the results of the strength-reduced composition
are theoretically equivalent, there may be small differences in
practice due to numerical errors, especially when using low
precision.  This is unlikely to be an issue, but, just in case,
allow disabling this optimization using the
GIMP_NO_PASS_THROUGH_STRENGTH_REDUCTION environment variable.