Apparently pbuffers are also a deprecated and unportable
way of offscreen rendering. They're not supported by
Chromium OpenGL implementation, which is what 3D-accelerated
VirtualBox uses.
This would also help a future OS X port.
Apparently GLX pixmaps are an antiquated and unsupported method
for offscreen rendering in OpenGL. Specifically this broke
software and VirtualBox accelerated rendering in Ubuntu trusty
and who knows which other systems.
This commit switches the rendering to use pbuffers instead,
while retaining all other logic. The pbuffers are fixed-size,
because reallocating pbuffers during a resize sometimes results
in segfaults in glXMakeCurrent despite apparently correct
API usage.
In principle, GTK3 is the way forward, and GTK2 is officially
deprecated, though still maintained. In practice however, GTK3
is often unbearably buggy; e.g. on my system, combo boxes
don't ever roll up in GTK3 windows. So I have added support
for both.
This required a few minor changes to the core, namely:
* GTK wants to know beforehand whether a menu item is a check
menu item or a regular one.
* GTK doesn't give us an easy way to execute something after
any event is processed, so an explicit idle timer is added.
This is a no-op on Win32.
* A few function signatures were const'ed, since GTK expects
immutable strings when converting to Glib::ustring.
