warn user when GIF disposal mode is not supported #8753

Job	Run time
Cross-compiling arm64	6m 41s
Cross-compiling arm64-sve	7m 24s
Cross-compiling arm64-lowprecision	6m 49s
Cross-compiling armhf	5m 36s
Cross-compiling i386	6m 16s
Testing arm64 shard 0	6s
Testing arm64-lowprecision shard 0	6s
Testing armhf shard 0	6s
Testing i386 shard 0	3m 46s
Testing arm64 shard 1	8s
Testing arm64-lowprecision shard 1	9s
Testing armhf shard 1	7s
Testing i386 shard 1	3m 48s
Testing arm64 shard 2	11s
Testing arm64-lowprecision shard 2	7s
Testing armhf shard 2	7s
Testing i386 shard 2	4m 4s
Testing arm64 shard 3	10s
Testing arm64-lowprecision shard 3	7s
Testing armhf shard 3	11s
Testing i386 shard 3	4m 0s
Testing arm64 shard 4	6s
Testing arm64-lowprecision shard 4	6s
Testing armhf shard 4	10s
Testing i386 shard 4	8s
Testing arm64 shard 5	6s
Testing arm64-lowprecision shard 5	10s
Testing armhf shard 5	7s
Testing i386 shard 5	10s
Testing arm64 shard 6	7s
Testing arm64-lowprecision shard 6	8s
Testing armhf shard 6	6s
Testing i386 shard 6	6s
Testing arm64 shard 7	8s
Testing arm64-lowprecision shard 7	7s
Testing armhf shard 7	7s
Testing i386 shard 7	7s
	51m 58s

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