Audio, Screen, and Webcam Recording Backends

[jonoomph]

This PR adds cross-platform recording support to libopenshot, including microphone, screen, webcam, and
supported system-audio capture. It also introduces higher-density peak/RMS waveform data, accurate live-
capture timestamps, a new Timer effect, and webcam layout properties.

Recording Support

• Linux/X11 ✅ V4L2 ✅ PulseAudio
• Linux/Wayland ✅ PipeWire portal ✅ V4L2 ✅ PulseAudio
• Windows ✅ GDI ✅ DirectShow ✅ WASAPI loopback
• macOS ✅ AVFoundation ✅ AVFoundation

Audio Recording

• Add a JUCE-based AudioRecorder
• Support WAV, FLAC, and MP3 output
• Provide live peak and RMS level monitoring
• Generate configurable peak/RMS waveform samples while recording
• Fix FFmpegWriter FLAC duration metadata
• Keep microphone audio in an independently editable file

Screen Capture

• Add Linux X11 screen capture
• Add Wayland capture through xdg-desktop-portal and PipeWire
• Support Wayland crop metadata and cropped window streams
• Drop invalid PipeWire frames safely
• Add Windows GDI screen capture
• Add macOS AVFoundation screen capture
• Support full-screen, window, and region bounds where available
• Interrupt blocking capture reads during shutdown

System Audio

• Add PulseAudio monitor capture on Linux
• Add WASAPI loopback capture on Windows
• Synchronize initial audio with captured video frames
• Preserve system-audio timing during frame writing
• Fix MinGW Core Audio GUID linking
• Report backend support so clients can disable unavailable controls
• macOS system-audio capture is not currently supported

Webcam Capture

• Add Linux V4L2 camera capture
• Add Windows DirectShow camera capture and COM enumeration
• Add macOS AVFoundation camera capture
• Support device-provided default capture modes
• Improve capture buffer cleanup and shutdown behavior

Timing and Synchronization

• Preserve source capture timestamps on live frames
• Preserve timing gaps instead of compressing dropped or delayed frames
• Support sparse video timestamps in FFmpegWriter
• Align microphone, screen, webcam, and system-audio recordings
• Add timestamp compatibility tests

Waveforms

• Add configurable waveform sample density
• Store both peak and RMS envelopes
• Preserve absolute signal levels instead of always normalizing quiet audio
• Fix recording waveform extraction and FLAC metadata handling

Clip Layout Properties

• Add animatable clip margin
• Add animatable corner radius
• Support inset and rounded webcam overlays
• Include JSON serialization and rendering tests

Timer Effect

• Add a configurable Timer effect
• Support count-up and count-down modes
• Provide text, font, color, background, outline, position, and timing controls
• Register the effect across C++, Python, Java, and Ruby bindings

Compatibility and Build Improvements

• Support older FFmpeg avdevice headers and input-format APIs
• Preserve compatibility with the existing macOS Python build environment
• Add required public headers and SWIG bindings
• Add optional Wayland/PipeWire build detection
• Link required Windows COM and multimedia libraries
• Install libavdevice-dev in GitHub Actions
• Expand Linux, Windows, and macOS build configuration

Validation

• Full libopenshot build succeeds
• All 515 unit tests pass
• Python bindings expose the new recorder, capture readers, settings, and Timer effect
• Manually verified Linux/X11 and Windows recording
• Manually verified Linux and Windows system-audio capture
• GitLab CI passes across the supported builders