VideoReaders need to be able to seek to specific frames

[jasoncorso]

This is a basic need that surprisingly has not arisen yet with the use of ETA, but there is no implemented support for video seeking. The underlying functionality in FFmpeg and OpenCV readers should already include this, which would just need to be wrapped.

[brimoor]

The way to seek now is via the frames argument of *VideoReader. But behind the scenes it reads frame-by-frame.

Since video readers support passing in arbitrary numbers of frame ranges of interest, the simplest optimization we could do is throw in an -ss flag to skip to the beginning of the first frame range and then read frame-by-frame from there.

[brimoor]

Scope amended based on #383.

From @jasoncorso:

VideoReader will need to support seeking formally in order for this to work as modules may need to go back and forth or at the very least "reopen" a video

[brimoor]

Scope back to just seek, in light of #384.