My previous commit broke the autotools build. Apparently when using
g_object_unref(), some C++ symbol leaked into libapppaint.a archive
library, hence the main binaries (e.g. gimp-2.99) could not be linked
without adding -lstdc++ flag:
> /usr/bin/ld: paint/libapppaint.a(gimppaintcore-loops.o): undefined reference to symbol '__gxx_personality_v0@@CXXABI_1.3'
> /usr/bin/ld: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: error adding symbols: DSO missing from command line
Not exactly sure why using this GLib function in particular caused this,
but let's just try another approach in order not to link the main binary
with C++ standard lib.
Instead let's manage all GeglOperation allocation in gimp-layer-modes.c
by adding a gimp_layer_modes_exit() function and some static array for
storing operation object of each layer mode.