Handle more (hopefully all) edge-cases when updating `opacity_target_old`:
* `MAPPING` transitions to `FADING` if new target is reached. Otherwise
no update.
* `FADING` clamps to current `opacity` if old target is too small.
* `UNMAPPING`/`DESTROYING` clamps to current `opacity` if old target is
too small.
* Add new field `opacity_target_old` to `struct managed_win` for
tracking relevant `opacity_target` changes.
* Smoothly fade blur-texture opacity on window opacity changes (based on
window opacity), when the window was or will be fully transparent
(`w->opacity ~< 0.004`).
* Fixed alpha-clipping of the blur-texture when using `inactive-opacity` or
repeatedly setting window opacity with large fade intervals (should fix#314).
