Add audio support and mute option for video playback

[sortedcord]

This pull request adds audio playback support for video splash screens in gsplash, allowing video files with audio streams to play sound unless the new --mute flag is provided. It introduces a new AudioPlayer implementation using FFmpeg and SDL2, updates the build system and main program logic to manage audio, and exposes audio-related command-line options.

Audio playback support for video splash screens:

• Added a new src/audio.c implementation and src/audio.h interface, providing the AudioPlayer struct and functions for initializing, decoding, and cleaning up audio streams using FFmpeg and SDL2 audio subsystems.
• Updated the Makefile to link against libswresample and include src/audio.c in the build.

Integration with main program:

• Modified src/gsplash.c to initialize SDL audio, detect video audio streams, and manage an AudioPlayer instance alongside video playback. Audio playback is started automatically unless the new --mute flag is set.
• Added --mute command-line argument to suppress audio playback and updated the help message accordingly.

Video/audio stream detection:

• Extended VideoPlayer in src/video.h and src/video.c to track both video and audio stream indices, allowing the main program to determine when audio playback is possible.

Other improvements:

• Cleaned up and clarified event handling and resource management in the main loop, including pausing audio when the splash window is hidden and ensuring proper cleanup.
• Update the interactive tests to include video background testing

These changes collectively enable synchronized audio/video splash screens in gsplash, with robust handling and user control over audio playback.