ass_strtod: skip leading zeros in mantissa
ass_strtod: skip leading zeros in mantissa

ass_strtod reads at most 18 leading digits of the mantissa.
This previously included zeros, even though they are not significant
digits, e. g. 0.000000000000000001e18 was converted to 0.0.
After this commit, leading zeros before and after the decimal point
will be skipped, so the above number will be correctly converted to 1.0.

ShiftMediaProject/libass,grigorig/libass,libass/libass,libass/libass,grigorig/libass,MrSmile/libass,xbmc/libass,ShiftMediaProject/libass,astiob/libass,xbmc/libass,MrSmile/libass,Akemi/libass,Akemi/libass,astiob/libass,Akemi/libass