Fixup 41caf2d for DeltaValue tables

Not that I can find any fonts having these... Had to construct one
to test.
Simplify converter mapping for DeltaValue
Move countVars into reader/writer objects

The countVars arguments are all unused now.  Cleanup coming.
Instead of countVars, pass down tableDict

This is the tableDict of current struct being loaded.  This does NOT
get propagated down.

Currently unused.
Use tableDict for DeltaValue lookbacks

Was abusing the set/getCount machinery before.
Only propagate counts that are used in sub-tables
Remove unused parameter
Minor