osc.lua: allow adding custom buttons

[guidocella]

Allow adding custom buttons without forking the whole OSC like in https://github.com/christoph-heinrich/mpv-quality-menu/blob/master/osc.patch

I tried to put these to the left of the cache text at first but the layout is determined at startup so you can't conditionally remove elements at runtime.

Emoji may not be perfectly aligned depending on the font used.

Example with:

custom_button_1_content=🔁
custom_button_1_mbtn_left_command=cycle-values loop-file inf no
custom_button_1_mbtn_right_command=cycle-values loop-playlist inf no

custom_button_2_content=🔂
custom_button_2_mbtn_left_command=cycle-values loop-file 1 no

custom_button_3_content=🔀
custom_button_3_mbtn_left_command=playlist-shuffle

[github-actions]

Download the artifacts for this pull request:

Windows

• mpv-i686-w64-mingw32
• mpv-x86_64-w64-mingw32
• mpv-x86_64-windows-msvc

macOS

• mpv-macos-13-intel
• mpv-macos-14-arm
• mpv-macos-15-arm