Restore overflowing for cells with non-default vertical alignment

Since the changes in a40acbb28e (Add CanOverflow function to
wxGridCellAttr, 2020-02-06), cells with non-default vertical alignment
didn't overflow any more, even if their horizontal alignment was
unchanged and still defaulted to left-aligned.

This was due to assuming that if the alignment of wxGridCellAttr itself
was different from wxALIGN_LEFT, it meant that it wasn't left-aligned,
which seems logical but is in fact false, as the alignment can also be
wxALIGN_INVALID, in which case the real alignment is taken from the
default grid attribute.

Fix this by using GetNonDefaultAlignment() to get the alignment value
effectively used and add a unit test, as well as an example in the
sample, showing that this now works correctly.

samples/grid/griddemo.cpp

@@ -614,6 +614,7 @@ GridFrame::GridFrame()
     grid->SetRowSize(10, 30);
     attr = new wxGridCellAttr;
     attr->SetBackgroundColour(*wxLIGHT_GREY);
+    attr->SetAlignment(wxALIGN_INVALID, wxALIGN_CENTRE);
     grid->SetRowAttr(10, attr);
     grid->SetCellValue(10, 0, "You can't resize this row interactively -- try it");