The existing code doesn't correctly handle the case where palettes partially overlap in the color record array. This changes the subsetting to only share entries in the color record array when palettes have the same first color index. Partially overlapping palettes will be converted to disjoint segments in the color record array.
Updates one of the color tests to use multiple palettes.
Also fixes fuzzer: https://oss-fuzz.com/testcase-detail/5568200165687296.
Garret Rieger