feat: Add a buffer fullness method (#3392)
feat: Add a buffer fullness method (#3392)

Closes #3389
shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player