Fix#2499, which see for details of an obscure problem affecting both PhpSpreadsheet and Excel. Add support for palette contained in workbook styles. This seems to be a very rare occurrence, so allow it only when the palette contains exactly 64 entries. If there are other possibilities, we'll presumably have a new workbook to guide us how to handle them. Also add some tests for specification of indexed color without palette, another rarity (no in-range examples amongst our current files). Also change one private static array, initialized once at run-time and never changed, to a constant.
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