Previously, we were always adding a #version 150 directive to shaders
when the GLSL version was higher than 1.2, which could cause compilation
to fail on platforms with GLSL versions between 1.2 and 1.5.

This fixes warnings that are emitted when building with the `-analyze`
flag enabled.

This fixes BadWindow errors from Expose events that may be processed after the window is destroyed.

On Linux, repaints are only issued if the number of received SHM
completion events matches the number of events initiated with
`xShmPutImage`.

Previously, destroying a GL context was calling `XSync (display, True)`
which cleared the event queue. Sometimes, this would cause an 'SHM
Completion Event' to be removed from the queue. This meant that the
number of received events would never match the number of initiated
events, and repainting would stop indefinitely.

By calling `XSync (display, False)` instead, we ensure that pending
events are kept in the queue, so any pending SHM completion event will
be found as usual during the next repaint timer callback.