frontend: Rename blending method labels

[SoprachevAK]

Description

Renames the blending method labels to describe their actual behavior:

• Default is renamed to Linear.
• SRGB Off is renamed to Nonlinear.

This is a UI-only terminology change. It does not modify rendering behavior, internal enum values, or serialized scene collection values.

The internal enum names (OBS_BLEND_METHOD_DEFAULT and OBS_BLEND_METHOD_SRGB_OFF) and serialized values are intentionally left unchanged. These names describe implementation-level render behavior and remain appropriate in code, while the new UI labels describe the result selected by the user. Keeping the identifiers unchanged also avoids unnecessary API and scene collection compatibility changes.

Motivation and Context

The existing labels describe implementation details rather than the behavior users select.

Default does not indicate that colors are blended in linear-light space. SRGB Off can also suggest that sRGB itself is disabled. In practice, that option disables linear-sRGB processing for the blending operation, causing encoded color values to be blended in nonlinear space. The color data does not stop being sRGB.

Linear and Nonlinear describe the observable blending behavior directly and avoid exposing an implementation-specific toggle in the UI.

How Has This Been Tested?

Manually tested on macOS 26.5.2:

• Opened the context menu for a scene item.
• Verified that the Blending Method submenu displays Linear and Nonlinear.
• Switched between both methods and verified that the selected state is displayed correctly.
• Saved and reloaded the scene collection and verified that the selected method is preserved.

The change only modifies English localization strings. Rendering and serialization code paths are unchanged.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.