And rename to QgsUnitTypes::RenderUnits. This enum is used in
much more contexts than just symbols, so QgsUnitTypes is a
better fit for it.
All methods which previously accepted QgsSymbolV2::OutputUnit
parameters or QgsSymbolV2::OutputUnitList parameters now take
QgsUnitTypes::RenderUnit or QgsUnitTypes::RenderUnitList
parameters respectively.