Jordan Russell's patch: only process mouse drags when we have the

Jordan Russell's patch: only process mouse drags when we have the
mouse capture, which in turn will only occur if the initial click
was in the PuTTY client area. Prevents mouse drag events without an
initiating click, and also prevents selection occurring at
unexpected moments such as if you move the mouse before releasing
the second click after double-clicking the title bar to maximise the
window.

[originally from svn r1515]