cmap14 subsetting code was not considering variation selectors in the input unicode set when deciding which variant glyphs to keep. This updates subsetting to only keeps variant glyphs if their variation selector code point is in the input unicodes set.
For https://github.com/harfbuzz/harfbuzz/issues/4911
NotoColorEmoji.ttf uses two cmap subtables
Format 14 | Platform ID 0 | Platform Encoding ID 5
Format 12 | Platform ID 3 | Platform Encoding ID 10
This combination results in the cmap table being dropped during subsetting despite being valid/required.
Include stdbool.h in hb-setset-test.h instead of in the individual
sources, if it is found; otherwise use a simplistic fallback for it if
it is not found.
Also declare variables at the top of the block, to build on pre-C99
compiliers.
