Add symbolic constant wxALIGN_INVALID and use it instead of -1 in wxGrid.

wxGridCellAttr was using literal -1 to indicate "no alignment" which wasn't
immediately obvious, use a new wxALIGN_INVALID (which has the same value)
instead.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62727 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/defs.h

@@ -1466,6 +1466,11 @@ enum wxDirection
 
 enum wxAlignment
 {
+    // 0 is a valid wxAlignment value (both wxALIGN_LEFT and wxALIGN_TOP use
+    // it) so define a symbolic name for an invalid alignment value which can
+    // be assumed to be different from anything else
+    wxALIGN_INVALID           = -1,
+
     wxALIGN_NOT               = 0x0000,
     wxALIGN_CENTER_HORIZONTAL = 0x0100,
     wxALIGN_CENTRE_HORIZONTAL = wxALIGN_CENTER_HORIZONTAL,