Fixes for cutting imported clips

A small follow-up to 2.5.0, fixing how imported videos behave after you change their speed or cut them.

Speed changes and cuts on imported videos stay accurate. Changing the speed of a video you added to the timeline and then cutting it no longer sends the later pieces to the wrong part of the video (or past its end). Every piece plays its own section, in the preview and in the exported file. A project where this happened in 2.4.2 or 2.5.0 may need its cut redone.
Appended recordings keep their cursor and camera in sync. When a recording of a different size is added to a project and then cut or sped up, the cursor, click effects and camera now follow each piece exactly.
The preview updates right away while paused. Changing a clip's speed or an image's duration with playback paused now shows the right frame immediately, and pressing Play runs at the new speed.
No stale frame between clips. Playing from one imported clip into the next no longer flashes a frame from the wrong moment before the right picture appears.

Screenify Studio v2.5.1