Fix transparency in webm videos

[CCharles99]

Launch Checklist

• Make sure the PR title is descriptive and preferably reflects the change from the user's perspective.
• Add additional detail and context in the PR description (with screenshots/videos if there are visual changes).
• Manually test the debug page.
• Write tests for all new functionality and make sure the CI checks pass.
• Document any changes to public APIs.
• Post benchmark scores if the change could affect performance.
• Tag @mapbox/map-design-team @mapbox/static-apis if this PR includes style spec API or visual changes.
• Tag @mapbox/gl-native if this PR includes shader changes or needs a native port.
• Tag @mapbox/gl-native if this PR disables any test because it also needs to be disabled on their side.
• Create a ticket for gl-native to groom in the MAPSNAT JIRA queue if this PR includes shader changes or features not present in the native side or if it disables a test that's not disabled there.

Description

Fixes #12662
Browser version: Chrome 148.0.7778.217 (Official Build) (64-bit)

Premultiplied alpha is correctly applied on the video texture's initial load, however, loading other textures would disable it and cause subsequent frames to render white at high transparency.

Setting premultiplied alpha to true before each video frame is rendered gives the desired result.

[github-actions]

Hey, @CCharles99 👋 Thanks for your contribution to Mapbox GL JS!

Important: This repository does not accept direct merges. All changes go through our internal review process.

What happens next:

1. A team member will review your PR here first
2. If it looks good, they will import it to our internal repository for further review
3. If approved, changes will be synced back here via our release process

Please respond to any review comments on this PR. For more details, see CONTRIBUTING.md.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}