X11: Fix GL related memory leak and crash

[Quant1um]

This PR fixes a memory leak caused by not releasing the GL context on X11 by calling glXDestroyContext, and fixes a BadDrawable error when closing the window in the middle of drawing by synchronizing a call to WindowHandle::close() with the event loop via channels.

Should fix #142

[micahrj]

I don't think the synchronization is sufficient here. The SyncSender will only block until the message is received, but we actually need WindowHandle::close to block until the process of closing the window is complete. I would recommend implementing this by storing the JoinHandle for the event loop thread in WindowHandle and joining it in WindowHandle::close.

If you don't mind, I think it would be ideal to move the window close synchronization code into a separate PR, since it's a more complex change and will probably need to go through some iteration. That way the change to add the glXDestroyContext call can get approved and merged right away.