The `gdk_window_ensure_native` call for window freeze and thaw
really is needed, but since it is incompatible with transparent
canvases, don't use freeze and thaw at all for those.
Meanwhile, repair the backing bitmap for both GTK+ 2 and 3
for a transparent canvas when a scale is in effect. And go
back to using X11 bitmaps for backing a canvas on GTK+ 3;
I'm not sure that's the right choice, but probably putting
the bitmap data on the X server instead of client is the
right thing.
Finally, restore GL bitmap support (partly by using X11 bitmaps
to back a canvas).
GL rendering to a canvas with a backing scale is not yet right,
either for GTK+ 2 or 3.
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.
