ffmpeg tutorial

by dranger

WIP Updating ffmpeg tutorial written by dranger.

This update has been tested using the following libraries versions:

ffmpeg version 4.3.1 Copyright (c) 2000-2020 the FFmpeg developers
  libavutil      56. 51.100 / 56. 51.100
  libavcodec     58. 91.100 / 58. 91.100
  libavformat    58. 45.100 / 58. 45.100
  libavdevice    58. 10.100 / 58. 10.100
  libavfilter     7. 85.100 /  7. 85.100
  libavresample   4.  0.  0 /  4.  0.  0
  libswscale      5.  7.100 /  5.  7.100
  libswresample   3.  7.100 /  3.  7.100
  libpostproc    55.  7.100 / 55.  7.100

Building using Clang version 11.0.3 and GCC 10.2.0 on macOS Catalina 10.15.7.
That will be tested on Linux soon.

Chapters updated:

• Introduction
• Tutorial 01: Making Screencaps
• Tutorial 02: Outputting to the Screen

Let's get started!