Check for NaN in fz_rendershade.
Check for NaN in fz_rendershade.

Fixes crash rendering chinesopera.pdf

darcs-hash:20080409170018-9beca-d40a155287a07aef1801915601bd16830d635359.gz

nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf,nqv/mupdf