Fix #12775: Change default texture to transparent

[gy1016]

Changes

This PR fixes #12775 by changing the default texture and cube map from white [255, 255, 255, 255]\ to transparent [0, 0, 0, 0].

Problem

When constructing a new Material, users may see a white flash during material loading. This is because the Update-loop-based image loading model that Material employs causes a brief period where the white default texture is visible.

Solution

Make the default texture transparent instead of white. This way, materials won't flash white into existence before loading.

Files Changed

• \packages/engine/Source/Renderer/Context.js: Changed defaultTexture and defaultCubeMap from [255, 255, 255, 255]\ to [0, 0, 0, 0]\
• \CHANGES.md: Added entry for this fix

Testing

• No test changes required as existing tests only check if defaultTexture is used, not its color value
• Manual testing recommended to verify no visual regressions

[github-actions]

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