Get the number of active morph targets for a given quality level. If no morph error data is available, or if no quality levels have been setup, the total number of morph targets is returned. If there is such data, the quality level will be clamped to be within a valid range. Finally, if the number of active morph targets specified by the quality level will be clamped within a valid range as well. So basically this method will always return a valid number of morph targets, whatever the input is.