warn user when GIF disposal mode is not supported #13534

Job	Run time
Ubuntu Build release	14m 25s
Ubuntu Build debug	5m 16s
Ubuntu Build asan	1m 47s
Ubuntu Build msan	4m 32s
Ubuntu Build tsan	1m 8s
Ubuntu Build scalar	3m 48s
Ubuntu Build lowprecision	14m 25s
Ubuntu Build coverage	1m 2s
Ubuntu Build release-nojpeg	4m 48s
Ubuntu Build release-lcms2	4m 53s
Ubuntu Build release-system-lcms2	5m 9s
Ubuntu Build release:minimal	5m 8s
Ubuntu Build release:gcc8	4m 2s
Ubuntu Build release:clang-7	4m 42s
MacOS Build release:osx	2m 39s
	1h 17m 44s

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