solmask: fix remove_islands for non-binary mask #1597

Job	Run time
macOS 15 (Intel, w/o python)	2m 26s
Ubuntu 22.04	3m 1s
Windows 2022	7m 56s
Ubuntu 24.04	5m 39s
AlmaLinux 8	4m 21s
Ubuntu 22.04 with Clang 13	4m 11s
Ubuntu 24.04 arm64	2m 34s
macOS 15	1m 45s
Docs build (Doxygen + Sphinx/Breathe)	1m 32s
	33m 25s

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