Add processing modes

[bmatherly]

Remove the previous GPU Effects option and replace it with multiple processing modes that enable higher bit depth and linear color processing.

Depends upon mltframework/mlt#1159

[Copilot]

Pull Request Overview

This PR replaces the binary GPU Effects option with a comprehensive processing mode system that supports multiple bit depths and color processing modes. The change introduces five processing modes: Native 8-bit CPU, Linear 8-bit CPU, Native 10-bit CPU, Linear 10-bit CPU, and Native 10-bit GPU/CPU.

Key changes:

• Introduced ProcessingMode enum and associated management functions
• Updated video widget configuration to handle different image formats based on processing mode
• Modified project saving/loading to store processing mode information

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/videowidget.cpp	Updated consumer configuration to set image format and color transfer based on processing mode
src/shotcut_mlt_properties.h	Added project processing mode property constant
src/settings.h	Added ProcessingMode enum and related method declarations, removed legacy GPU setter
src/settings.cpp	Implemented processing mode management with legacy GPU mode migration
src/mltcontroller.h	Added processing mode support to controller
src/mltcontroller.cpp	Integrated processing mode into project loading/saving and controller initialization
src/mainwindow.ui	Replaced single GPU action with processing mode menu and actions
src/mainwindow.h	Updated method signatures for processing mode
src/mainwindow.cpp	Replaced GPU handling with comprehensive processing mode management
src/main.cpp	Updated command line GPU option to use new processing mode
src/docks/encodedock.cpp	Added processing mode support to encoding configuration

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}