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.

tests/controls/gridtest.cpp

@@ -925,6 +925,14 @@ TEST_CASE_METHOD(GridTestCase, "Grid::GetNonDefaultAlignment", "[grid]")
     attr->GetNonDefaultAlignment(&hAlign, &vAlign);
     CHECK( hAlign == wxALIGN_RIGHT );
     CHECK( vAlign == wxALIGN_CENTRE_VERTICAL );
+
+    // This is only indirectly related, but test here for CanOverflow() working
+    // correctly for the cells with non-default alignment, as this used to be
+    // broken.
+    m_grid->SetCellAlignment(0, 0, wxALIGN_INVALID, wxALIGN_CENTRE);
+    attr = m_grid->CallGetCellAttr(0, 0);
+    REQUIRE( attr );
+    CHECK( attr->CanOverflow() );
 }
 
 TEST_CASE_METHOD(GridTestCase, "Grid::Editable", "[grid]")