Add Travis CI testing enviroment

[AkashiSN]

Summary

• A CUDA testing environment with Docker and native OSX testing have been added to Travis CI.
• Changed Dockerfile to use a multi-stage build.
• Added Dockerfile for building CUDA into ffmpeg with nvidia/cuda container. (cuda-ubuntu.dockerfile)
• The indentation was unified to Tab and re-indent.
• Changed the config option (--enable-*) to be added in each build section (CONFIGURE_OPTIONS).
• Removed some config options because they were redundant.

About CUDA

Since the version of CUDA published by nvidia/cuda is 11.1 in ubuntu 20.04, and the target architecture is very old (SM30) in ffmpeg's default nvccflags and completely removed in CUDA11, the target architecture is SM52 added a flag to change to. (ref: https://arnon.dk/matching-sm-architectures-arch-and-gencode-for-various-nvidia-cards/)
--nvccflags=-gencode arch=compute_52,code=sm_52

Backwards compatibility exists (just not optimal), so you only need to change this flag when using older graphics cards or if you want to optimize for a particular graphics card.

Removed some config options

• --enable-runtime-cpudetect : Enabled by default. (https://github.com/FFmpeg/FFmpeg/blob/6b6b9e593dd4d3aaf75f48d40a13ef03bdef9fdb/configure#L108)
• --enable-avfilter : Enabled by default. (https://github.com/FFmpeg/FFmpeg/blob/6b6b9e593dd4d3aaf75f48d40a13ef03bdef9fdb/configure#L134)
• --enable-filters : Enabled by default.
  (https://github.com/FFmpeg/FFmpeg/blob/6b6b9e593dd4d3aaf75f48d40a13ef03bdef9fdb/configure#L186)

Add config options

• --enable-libxvid : Adding this option resulted in dynamic linking, so the code to remove the dynamic library was added.

Sincerely,

[markus-perl]

Hi AkashiSN, Thank you for all this improvements and the high quality of the pull request. I mode some small improvements and merged the commit.

Best,
Markus