v4l2videodec: Implement a minimal jpeg parser

Submitted by Nicolas Dufresne @ndufresne

Link to original bug (#766076)

Description

Until https://bugzilla.gnome.org/show_bug.cgi?id=626531 get addressed, it is required that all jpeg decoder can at least split the jpeg data into single frame. For this reason, v4l2videodec need to implement parsing when jpeg is selected.