Improvements in parsing TTF Fonts

[ashutosh1211]

You have written very good parser, it works.
While parsing TTF font i observed few issues or missing code as follow.

1. While parsing lookups of type 7 , offset is not added.
   

   opentype/src/tables/gsub.js

   Line 250 in 31d5f3c

   data = gsub.LookupType(buffer, extensionLookupType, extensionOffset);

   
   Here we need to add offset of current lookup.

2. Do not read markFilteringSet if LookupFlag is not set.
   

   opentype/src/tables/common.js

   Line 138 in 31d5f3c

   var markFilteringSet = buffer.read(Type.USHORT);

   
   Reference: https://docs.microsoft.com/en-us/typography/opentype/spec/chapter2#lookupTbl