Add 'decoder' parameter to VideoFileClip and ffmpeg_reader - v2 (dev)

[BackrndSource]

Same as #2229 but for v2 (dev)

Fixed bug: - When importing a .webm video with alpha channel, transparency are not working.

Solved issue: - #2008

References: - https://www.reddit.com/r/ffmpeg/comments/fgpyfb/help_with_webm_with_alpha_channel/

"FFmpeg's native VPx decoders don't decode alpha. You have to use the libvpx decoder to preserve transparency in .webm videos. "

To preserve the alpha channel also needs to set has_mask parameter to True.

Example of use:

VideoFileClip("video.webm", has_mask=True, decoder="libvpx-vp9")