Windowless richedit control will not be able to call GetCapture without
a handle to the host window (and there is no ITextHost_TxGetCapture
method), but there is a ITextHost_TxSetCapture method available for
setting and releasing the capture on the mouse.  This means that the
richedit control will need to keep track of whether it has captured the
mouse or not to implement windowless richedit controls.