This helps to ensure that a base class that changes underneath us
would not leave any overridden functions hanging.
This already highlighted some questionable use of GTKMM's API,
which were also fixed in this commit.
This is a high-SNR warning that's enabled by default on MSVC and
it has highlighted some bugs in glhelper.cpp (that are also fixed
in this commit).
Unfortunately GCC does not have an equivalent for that warning,
and -Wconversion is very noisy.
Without resources, it makes no sense anymore to keep these in
separate subdirectories: unixutil* is shared between Cocoa
and GTK ports, and gloffscreen* should be shared between all ports
(but it's still platform-specific).
