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.

src/generic/grid.cpp

@@ -647,9 +647,17 @@ wxGridFitMode wxGridCellAttr::GetFitMode() const
 
 bool wxGridCellAttr::CanOverflow() const
 {
-    int hAlign;
-    GetAlignment(&hAlign, NULL);
-    return GetOverflow() && (hAlign == wxALIGN_LEFT);
+    // If overflow is disabled anyhow, we definitely can't overflow.
+    if ( !GetOverflow() )
+        return false;
+
+    // But if it is enabled, we still don't use it for right-aligned or
+    // centered cells because it's not really clear how it should work for
+    // them.
+    int hAlign = wxALIGN_LEFT;
+    GetNonDefaultAlignment(&hAlign, NULL);
+
+    return hAlign == wxALIGN_LEFT;
 }
 
 // GetRenderer and GetEditor use a slightly different decision path about