Added option to force 4:3 aspect ratio

[vmzhivetyev]

Problem

Setup:

1. SkyZone sky04x v2 (not PRO) with 4:3 screen
2. Walksnail VRX
3. Walksnail Micro V2 Cam with 4:3 sensor

Situation:

• Sky04x goggles have 4:3 display but they get hdmi input in 16:9 format and scale it to fill the screen.
• In this case you have to use 4:3-FULL display mode in Walksnail VRX settings which also scales image coming from camera to occupy full screen. Otherwise the image would appear stretched in the goggles.
• Now you have 16:9 DVR recording done by the VRX but the camera image is stretched in width.
• To fix it you have to rescale the video bringing the aspect ratio back to 4:3.

Solution

Added a new checkbox which enables rescaling the final video (after overlaying the OSD) to 4:3 aspect ratio thus effectively shrinking it horizontally. The added ffmpeg option -aspect doesn't really change final resolution rather just specify pixel aspect ratio in the mp4 container.