Add support for videos as background images

[OneTrickSupport]

Summary

Adds support for using short videos (mp4/mov) as terminal background images, addressing the long-standing request for animated backgrounds (GitLab issue #7920).

• The Settings > Profiles > Window file picker accepts videos alongside images, validates that the file is playable, previews the first frame, and shows a permanently-silenceable notice about the energy cost of high-resolution / high-frame-rate files.
• Videos play muted in a seamless loop (AVPlayerLooper) and respect blend, transparency, and all four scaling modes (Tile falls back to Fill).
• Works with both renderers:
  • GPU renderer: iTermBackgroundImageRenderer wraps each frame's pixel buffer as a BGRA Metal texture via CVMetalTextureCache (zero-copy). The update cadence treats a playing video like an inline animated GIF so frames draw at display rate.
  • Legacy renderer: an AVPlayerLayer in iTermImageView plays the video; playback position carries over seamlessly when the renderer switches because both paths share one player.
• iTermImageWrapper owns a single shared muted AVQueuePlayer + AVPlayerItemVideoOutput per video and loads a poster frame for image-only consumers (blending, tiling, snapshots). Consumers hold playback interests, so decoding pauses when nothing is visible and split panes showing the same video share a single decoder.

Test plan

Manually verified on macOS 26 (Apple Silicon), Xcode 26.6:

• Picking mp4/mov via the picker; picker still accepts images and folders; corrupt-file warning still works
• Playback under the GPU renderer (confirmed active) and under the legacy renderer (transparency enabled)
• Split panes: shared decoder, synchronized playback; per-pane and whole-window background modes
• Blend slider and all scaling modes live-update
• Playback pauses when the window is hidden/miniaturized
• Removing the video restores normal behavior

Known limitations

• Drag-and-drop of a video onto the settings image well is not supported (picker only) since NSImageView filters non-image drags
• The Metal path samples raw BGRA without color-space conversion, so colors can differ very slightly from the legacy path
• The non-monochrome manual drawing path (e.g. minimum contrast) shows the static poster frame

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