[truetype] Support reading 32bit/16bit VarStore deltas

* include/freetype/internal/ftmmtypes.h (FT_ItemVarDelta): Define type
to be used for delta arrays, upgrade to FT_long.
* src/truetype/ttgxvar.c: Adhere to long_words bit and read either
Short/Byte pairs or Long/Short pairs, as defined by spec. For better
readability, define macro for repetitive read code.
13 jobs for 32bitVarStoreDeltas in 12 minutes and 23 seconds (queued for 3 seconds)