@remotion/effects: Keep halftone linear gradient background

[JonnyBurger]

Fixes #7734.

In solid color mode, halftoneLinearGradient() now composites dots over the incoming source instead of clearing the background between dots.

Validated with bun run build, bun run formatting, bun test src/test/effect-params.test.ts, and bunx turbo make --filter="@remotion/effects".

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Project	Deployment	Actions	Updated (UTC)
bugs	Ready	Preview, Comment	May 28, 2026 5:23am
remotion	Ready	Preview, Comment	May 28, 2026 5:23am

[pullfrog]

Reviewed changes — Fix for halftoneLinearGradient() in solid mode to preserve the background between dots by compositing over the incoming source instead of outputting transparent pixels.

• Add sourceOver() GLSL helper — Standard premultiplied-alpha source-over compositing in the fragment shader.
• Composite dots over background in solid mode — In colorMode: 'solid', dots are now blended over texColor rather than multiplied by coverage alone, which previously cleared the background.
• Update docs — Clarify that solid mode composites over the previous effect in the chain.

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Note: 1 inline comment(s) dropped because they did not anchor to lines inside the PR diff:

• packages/effects/src/halftone-linear-gradient.ts:299-300 (RIGHT) — line 300 (RIGHT) is not inside a diff hunk

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