Fix premultiplied sRGB browser rendering

[SoprachevAK]

Description

Uses the scene item's linear-sRGB state to select the correct rendering path for premultiplied browser textures:

• Linear blending uses DrawSrgbDecompressPremultiplied.
• Nonlinear blending draws the texture without conversion.

Also sets OBS_SOURCE_INITIAL_BLEND_METHOD_SRGB_OFF so new browser scene items initially use nonlinear blending.

Depends on obsproject/obs-studio#13797, this fixes #469.

Motivation and Context

This is the obs-browser companion to obs-studio#13797, which contains the core rendering changes, migration logic, and full motivation.

Chromium composites content in nonlinear sRGB, so browser sources should initially use nonlinear blending while still providing a correct linear rendering path when selected by the user.

How Has This Been Tested?

Manually tested on a MacBook Pro with an Apple M5 Pro running macOS 26.5.2, together with the changes from obs-studio#13797.

Verified both linear and nonlinear blending with opaque and semi-transparent content over transparent, black, and white backgrounds. The test page, comparison screenshots, and detailed results are included in the main PR.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

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.

• I have used AI tooling in the creation of this PR.
  • AI tooling was used for research and autocomplete assistance. I deliberately reviewed, understood, and verified every submitted line of code, and personally evaluated all architectural decisions and trade-offs.