Remove call to XGrabKeyboard from XOpenGLRenderWindow.

This call locks users into fullscreen paraview server windows,
disabling alt-tab, etc.

Change-Id: I71ea8bd354bf301d0462f4ce82683eaf3aca64d5
Merge topic 'NoXGrabKeyboard' into master

a5d9e847 Remove call to XGrabKeyboard from XOpenGLRenderWindow.