Fix for 4132872 inefficient export logic.

Also fixes 3371103 Transitions between images takes a long time
to generate.

Redundant transcoding is removed. Decode encode will be done only
once per clip.
For images, conversion to 3gp clip is removed (except for generating
Kenburn image clip).

Change-Id: Id722cd0296641bd643eb5c63f4d8028716259c01