Fix banding and discoloration issue in 3D scene

[iche033]

🦟 Bug fix

Summary

The MinimalScene uses the GammaAdjust effect for gamma correction otherwise the scene will appear darker than it should when using ogre2 + OpenGL. However, it was found that caused caused banding and discoloration issues in the 3D scene, see screenshots here. This PR removes the GammaAdjust QML effect and instead applies a workaround in the OpenGL RHI to convert the input ogre2 render texture from the sRGB format to linear RGB format before passing it to Qt. The scene now display the correct colors but without banding issues.

Note this may not be necessary in Qt 6 as I see that there is a new class and API to set the flags of its RHI texture to sRGB color space

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[jennuine]

Looks good, whenever we migrate to Qt6 we'll need to remember to try w/o the workaround before we forward port the code. Thanks Ian.

[iche033]

replaced by #630