Improve the performance on searching blanks

After the change of d6a5cc665a1d7e91332944353e92c83ad114368c
we have a lot of code points in FcBlanks. doing the linear search
on the array isn't comfortable anymore.
So re-implementing FcBlanksIsMember() to use the binary search.

Figuring out how much improved after this change depends on
how many fonts proceed with fc-cache say though, it's about 20 times
faster here on testing. which sounds good enough for
improvement.
1 file changed