warn user when GIF disposal mode is not supported #8540

Job	Run time
.deb packages / ubuntu:24.04	26m 16s
Release linux x86_64 static	5m 35s
.deb packages / ubuntu:22.04	23m 13s
.deb packages / ubuntu:20.04	15m 3s
.deb packages / debian:bullseye	12m 57s
.deb packages / debian:bookworm	14m 0s
.deb packages / debian:trixie	1m 33s
.deb packages / debian:sid	12m 30s
Windows Build (vcpkg / x86-windows-static)	10m 22s
Windows Build (vcpkg / x64-windows-static)	18m 42s
Windows Build (vcpkg / x86-windows)	7m 51s
Windows Build (vcpkg / x64-windows)	10m 25s
	2h 38m 27s

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