[mathtext] fix windows warning about size_t -> Font_t conversion
[mathtext] fix windows warning about size_t -> Font_t conversion

Original messages (all the same):
warning C4267: 'argument': conversion from 'size_t' to 'Font_t',
possible loss of data 
olifre/root,olifre/root,karies/root,olifre/root,karies/root,olifre/root,karies/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root,root-mirror/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,karies/root,karies/root,root-mirror/root,karies/root,karies/root,root-mirror/root,karies/root,olifre/root,olifre/root,olifre/root,root-mirror/root,olifre/root