The CLEAR blend mode seems to be an obvious addition. The case for ALPHA_MAX is a bit more difficult to justify, as it seems like a special-purpose mode. You need this particular blend mode, but the next developer might need COLOR_DODGE_WITH_ALPHA_MAX or LIGHTEN_WITH_ALPHA_ATOP.
Maybe we can prevent a combinatorial explosion by separating color blending and alpha blending. Right now, all blend modes listed in BlendMode use one of three alpha blending modes: "source over", "source atop", or "add". We can add "max" to this list, and have a new AlphaMode enumeration with the following constants: DEFAULT, SRC_OVER, SRC_ATOP, ADD, MAX. The value DEFAULT means: use the alpha mode that is specified for the BlendMode. For example, BlendMode.SRC_ADD will use AlphaMode.SRC_ADD; similarly, BlendMode.SCREEN will use AlphaMode.SRC_OVER. But now, you can change that! Your special-purpose mode would be BlendMode.SRC_OVER + AlphaMode.MAX. My special-purpose mode will be BlendMode.LIGHTEN + AlphaMode.SRC_ATOP.
