The main advantage of GTK+ 3 is better support for HiDPI
displays. If GTK+ 3 libraries are not available or if the
`PLT_GTK2` environment variable is defined, GTK+ 2 is used
as before.
From the ARB_create_context_profile standard:
On failure glXCreateContextAttribsARB returns NULL and generates an
X error with extended error information.
Some NVIDIA drivers on Linux won't return a core profile. When they don't,
they're supposed to return NULL as well as generate an X error, but they
only generate an X error and return a non-NULL, garbage context. This is
not only nonstandard behavior, but lame. It's a major pain to check for X
errors. But whatever, NVIDIA, you big old gorilla.
This may or may not fix segfaults some people are having with Pict3D. If
it does, they won't have to use (current-pict3d-legacy? #t) anymore.
