vcl: since we share Bitmaps across all GLContexts simplify lifecycle.

~X11SalVirtualDevice() was destroying X resources referred to by
OpenGLSalBitmap's mpContext, which were subsequently accessed by
Bitmap::AcquireReadAccess on the floating Bitmap. Better to use the
default window's GLContext for all bitmap operations.

Change-Id: I9009980e791cff1a1f36d626592d72c7a32efd39