Revert "film strip: try to be thread safe while switching images."

This reverts commit 683f827b4a64de71c6c3598c4c2fbcec35ed84ac.

didn't quite solve it.
pixelpipe: introduce mutices for pipelines while develop_t uses them.
darkroom: lock pixelpipes before destroying them.

should fix crashes/hangs when pressing spacebar like crazy.