[iOS] Menu UI invisible but clickable after Vibrant Visuals brightness adjustment

Description of the Issue

When adjusting the Vibrant Visuals brightness slider on iOS, the global gamma_calibration config value is incorrectly overwritten by the brightness percentage. This causes all menu UI elements to become fully invisible but remain clickable. The bug does not manifest immediately; it requires a render pipeline reload to take full effect, and the config value is only flushed to disk when the game is backgrounded or closed.

Steps to Reproduce:

1. Verify that gamma_calibration is set to default 1.0 in options.txt before launching the game.

2. Launch Minecraft Bedrock on iOS. Main menu UI renders normally.

3. Go to Settings → Video, switch graphics mode to Vibrant Visuals.

4. Adjust the Vibrant Visuals brightness slider to a value other than 100% (e.g. 92%).

5. At this point, main menu UI remains mostly normal; only the 3D character preview may show minor rendering issues. The bug is not fully visible yet.

6. Enter any saved world and stay for a few seconds.

7. Do either of the following:

   • Return to the main menu from the world

   • Put the game in the background / lock the screen and wait 1–2 minutes

8. Resume the game or relaunch the app, then view the main menu, world select screen, or character creator.

Expected Result:

All UI buttons, text, and panels render normally. Only the 3D scene brightness should change. The gamma_calibration value should remain at 1.0, as it is a global screen calibration parameter and should not be modified by the in-game brightness slider.

Actual Result:

Nearly all menu UI elements disappear completely; only the background image remains. Buttons and interactive areas are still fully functional and clickable, just invisible.
Checking options.txt confirms that gamma_calibration has been incorrectly changed from 1.0 to match the brightness percentage (e.g. 92% → 0.92).
The full UI invisibility only appears after the render pipeline is reloaded. The incorrect value is not written to disk immediately; it only persists in memory until the game is backgrounded or closed.

Root Cause Analysis:

The Vibrant Visuals brightness control is intended to adjust only 3D scene exposure. However, on the iOS Metal render pipeline, the slider value incorrectly overwrites the global gamma_calibration parameter.
This parameter controls gamma calibration for the entire render pipeline (including all UI layers) and should remain at 1.0 by default.
The value is first modified in memory and only flushed to options.txt during app suspension / exit. The full visual glitch only triggers when the render pipeline reloads and applies the abnormal gamma value to the UI layer.

Additional Notes:

• This issue is exclusive to iOS/iPadOS with the Metal renderer.

• The severity depends on how far the brightness is from 100%; smaller deviations may only affect 3D elements, not solid UI panels.

• Manually setting gamma_calibration back to 1.0 in options.txt completely resolves the issue.