Review: Context Bugfix

Jeremy Selan <jeremy...@...>

This fixes two major bugs in OCIO::Context.

First, Context.getStringVar didnt work due to an incomplete definition
of the Compare funtion. (The consequence being that keys would be
sorted by string length, but for strings of equal length the wrong
value would be returned.).

Also, setStringVar (used in the Nuke context overrides) had no effect
due to a bug in the use of multimaps.

These have been verified to work now.

-- Jeremy