imwrite: implement DisposalMethod option. * __magick_read__.cc (F__magick_write__): set gifDisposeMethod() when writing an image to implement the DisposalMethod option. Set all the options in a single option rather than a loop per option. (init_reverse_disposal_methods): new method to return a reversed std::map of the disposal methods. (init_disposal_methods): move up, to be closer to the new init_reverse_disposal_methods() * private/__imwrite__.m: input check for the new option DisposalMethod and set its default. * imwrite.m: document new option.

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