video_output: fix blend filter being opened in loop
Logs:
main generic debug: looking for video blending module matching "any": 1 candidates
main generic debug: using video blending module "blend"
main blend debug: removing "video blending" module "blend"
main generic debug: looking for video blending module matching "any": 1 candidates
main generic debug: using video blending module "blend"
main blend debug: removing "video blending" module "blend"
Test scenario:
./vlc --vout=dummy sample_with_subpicture
Regression from 1caf1afa.