Improve the code by 5d3ff05615 ,
issues are found by Behdad Esfahbod and Werner Lemberg.
* src/cache/ftcbasic.c (FTC_ImageCache_Lookup): Replace
a subtraction to check higher bit by a bit operation,
and cpp-conditionalize for appropriate systems. Add better
documentation to the comment.
(FTC_ImageCache_LookupScaler): Ditto.
(FTC_SBitCache_Lookup): Ditto.
(FTC_SBitCache_LookupScaler): Ditto.
5ad845771a