Feature/alpha mask filter

[InnoLuke]

Based on https://github.com/MasayukiSuda/ExoPlayerFilter/pull/42

• Position of alpha mask within the video is a configurable parameter
• Adds the ability to handle transparency in EPlayerRenderer
• Adds a hook for GlFilters to modify videoAspect
• Adds a subclass of EPlayerView - EPlayerTranslucentView. EPlayerTranslucentView allows for video transparency, without breaking EPlayerView
• Sample is untouched

Migrated from https://github.com/MasayukiSuda/ExoPlayerFilter/pull/48 to allow for continued development

[MasayukiSuda]

GlFilterGroup is broken. Please fix.

[damikdk]

Hi, thanks for the amazing fork. Is it possible to finish this?

2022, Android still can't play transparent videos without your code