Fixed issue reported in previous commit. The cause was that for a wxGCDC setting the clip region to a zero sized rectangle results in clipping everything. The wxWidgets documentation states this should be equivalent to removing the clipping region altogether. This is a problem because wxDC::GetClippingBox returns a zero size rectangle if there is no clipping, therefore if we have no clipping then checking the clipping box, setting it, then restoring it to its original value results in clipping everything if the original state was clipping nothing.
Fixed issue reported in previous commit. The cause was that for a
wxGCDC setting the clip region to a zero sized rectangle results in
clipping everything. The wxWidgets documentation states this should be
equivalent to removing the clipping region altogether. This is a problem
because wxDC::GetClippingBox returns a zero size rectangle if there is
no clipping, therefore if we have no clipping then checking the clipping
box, setting it, then restoring it to its original value results in
clipping everything if the original state was clipping nothing.

This is clearly a wxWidgets bug. I have worked around it by checking for
a zero sized box and performing the correct action.

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