Port Rachana to mlm2 opentype spec while keeping mlym compatible.

This is a big change to introduce new mlm2 opentype spec changes
at the same time keeping backward compatibility with mlym spec.
The changes introduced are:
    1. Introduce new 'pref' (Pre Base Forms) lookup table for mlm2 'Virama+Ra'
    2. Renumber other lookup tables and include mlm2 tag
    3. Add 'pstf' (Post Base Forms) lookup for mlm2 'Virama+Ya' and 'Virama+Va'
    4. Add 'blwf' (Below Base Forms) lookup for mlm2 'Virama+La'
    5. Add 'pres' (Pre Base Substitutions) rules for mlm2 'Conjunct+Virama+Ra'
    6. Now 'Ra'kar has both 'pref' and 'pstf' lookup on the 'pres' rules. This
       causes issues with 'Cons1+Cons2+Virama+Ra' to be rendered with Ra on Cons1
       instead of correctly with Cons2. This is caused by first mlm2 'pref' applying
       reorders to 'Cons1+Ra-kar+Cons2' and the subsequent 'pres' rule for mlym
       renders Ra-kar to the base Cons1.
       This is fixed by moving mlym Ra 'pres' (Pre Base Substitution) rules to
       mlym Ra 'psts' (Post Base Substitution) rule. Tested OK with Windows XP SP3.
    7. Bump the version to 6.0
Tested with Windows XP SP3 (for mlym), Windows 7 (for mlm2) and Harfbuzz.

Signed-off-by: Rajeesh K Nambiar <rajeeshknambiar@gmail.com>