Manual BT.709 conversion
Revert "Correct ASS focal length to width*4/3"

This reverts commit 4fa2a01e1e2dea830d18fffd80f2fc3702e546ff.
Merge branch 'dev-bt709'
Fix mistype

... in commit a145947772363f95948f84eac9b4f39f5f1429c9