zoom and move

[HT-7]

Every image viewer I can think of has the ability to zoom and move within a picture. I suggest celluloid to implement this feature for videos.

Since a video can be panned, there needs to be a point of reference, so a rectangle that represents the current field of view within a rectangle that represents the video's full field of view needs to be displayed.

Samsung's mobile video player has this ability since the early 2010s, and VLC Media Player too, however, the zooming of VLC media player needs to be activated first and can only be changed by dragging the rectangle in the reference area on the top left, not by dragging in the video directly. Celluloid needs to support it through direct dragging, but a reference area should still be shown while zoomed in so the orientation is not lost.

In addition, when dragging the zoomed-in video, the video needs to move faster than the mouse cursor so less mouse movement is necessary, for the same reasons as stated at xviewer issue #186. In fact, Samsung's video player did this for years.