Added GraphicsPathCommand.CUBIC_CURVE_TO

The command `GraphicsPath.Command.CUBIC_CURVE_TO` wasn't in the flash externs.
While everywhere else the cubicCurveTo() is a flash11 feature, in the Adobe documentation http://help.adobe.com/en_US/FlashPlatform/reference/actionscript/3/flash/display/GraphicsPathCommand.html#CUBIC_CURVE_TO this particular field says that it works on a flash 10 runtime so I simply added it without `@:require(flash11)`

std/flash/display/GraphicsPathCommand.hx

@@ -4,6 +4,7 @@ extern class GraphicsPathCommand {
 	public static inline var LINE_TO = 2;
 	public static inline var MOVE_TO = 1;
 	public static inline var CURVE_TO = 3;
+	public static inline var CUBIC_CURVE_TO = 6;
 	public static inline var WIDE_LINE_TO = 5;
 	public static inline var WIDE_MOVE_TO = 4;
 	public static inline var NO_OP = 0;