ACTION-188: Update spec for flags, weith glyf and loca treated specially for historical reasons

Action 188: allow null transform for glyf and loca
- Table directory description was updated to say that the flags value (bit 6 and 7) need to be relied on in determining whether the a modifying transform was applied, and consult the combination of the table tag / transform version number to determine the specifics;
- Description of the flags bits 6 and 7 was updated to add null transform for glyf and loca tables (transform version 3);
- Removed explicit references to transformed table tags in table directory to make it open for future extensions;
- Added references to null transform in sections 5.1 (glyf) and 5.3 (loca) transform descriptions. In addition, the loca table transform is now defined as “conditional mandatory”, i.e. it is mandatory if glyf table is transformed but both tables can be presented in their original shape with null transform. There is an additional FF conformance statement conform-transformedLocaMustAccompanyGlyf that needs a new test be defined;