Handle 2D/3D mismatches in the player (#2409).

Handle 2D/3D mismatches in the player (#2409).

Previously if there was 2D content in a 3D project (or vice versa)
the player would emit the video anyway and the encoder would have
to deal with it.

Since previously the FFmpeg encoder did not deal with this quite
right, it seems to make sense to move the logic into the player
so that neither encoder has to worry about it.