This change fixes a bug where micview would hang when the user presses 'q' to
exit, and you'd have to kill -9 the process to make the window go away.

On Linux, with at least the radeon graphics driver, glutCreateWindow appears to
spawn several child threads for shaders, a disk cache, and something else:

si_shader:0
si_shader:1
si_shader:2
si_shader:3
disk_cache:0
radeon_cs:0

These threads appear to keep the process as a whole from exiting when
pthread_exit() is called.  The solution is to call glutDestroyWindow(), which
will cause glutMainLoop() to return, which causes main() to return cleanly.

Signed-off-by: mertg <[email protected]>
Reviewed-by: Benn Snyder <[email protected]>