Fix discrepancy between different ways of measuring text extents under Mac.

Fix discrepancy between different ways of measuring text extents under Mac.

wxGraphicsContext::GetTextExtent() didn't round the returned double result to
int but truncated it instead and so returned different extent than
GetPartialTextExtents() which did round it up.

Moreover, wxGraphicsContext::GetPartialTextExtents() didn't round it up
correctly: it wrongly added 0.5 to the value still stored as double and which
was hence rounded up (correctly, this time) when converted to int in
wxDC::GetPartialTextExtents().

These two errors combined to produce difference of up to 2 pixels between the
last offset returned by wxDC::GetPartialTextExtents() and the total string
extent returned by wxDC::GetTextExtent() which thoroughly confused the code in
wxControlBase::DoEllipsizeSingleLine() (and probably not only there).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62580 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775