Similar to commit 2c0cfdefbb9d1 for the SDL2 winsys, the GLX and EGL
window systems need to bind the dummy surface or drawable when the
currently bound onscreen is destroyed so that there will always be a
valid context bound.
Previously I got the idea that this would not be necessary on GLX
because the documentation for glXDestroyDrawable states that the
drawable won't actually be destroyed if it is currently bound until it
becomes unbound. However it doesn't say what happens if the underlying
X window is also destroyed and after testing it seems this causes a
segfault in Mesa in GLX and an XError for EGLX.
Reviewed-by: Robert Bragg <robert@linux.intel.com>
(cherry picked from commit 4a464eec8c5b5832b9fd6b69746ab4ab36229182)
1e00ff268e