gimp_layer_get_effective_mode() returns the actual layer mode,
blend space, comosite space, and composite mode used for the
layer's mode node, allowing them to be different from the values of
the corresponding layer properties.  The aim is to allow us to
replace expensive layer configurations with cheaper but equivalent
ones transparently.  This is used in the next commit to replace
pass-through groups with normal groups under certain conditions.

The effective values are computed by the new
GimpLayer::get_effective_mode() virtual function.  The default
implementation provided by GimpLayer returns the corresponding
layer properties as-is (replaceing AUTO with concrete values).
Subclasses can override this function, providing more
sophisticated logic.