mi: Use GC clipregion instead of drawable in miDoCopy/miCopyArea

As discussed in issue #829 the "mi: Shortcut miDoCopy/miCopyArea based
on clipList" change leads to pointer-trails (area under pointer not
restored when it moves) when using a software cursor.

Checking pGC->pCompositeClip instead of pDstDrawable->clipList fixes
this problem.

Fixes: #829
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
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