Updated fragment shader sample to use uniform-by-name accessor

[gaaclarke]

Do not land until the following are on stable:

• impeller: allows access of float uniforms by name flutter#176728
• Implements uniform-by-name for web flutter#176980

Pre-launch Checklist

• I read the Flutter Style Guide recently, and have followed its advice.
• I signed the CLA.
• I read the Contributors Guide.
• I have added sample code updates to the changelog.
• I updated/added relevant documentation (doc comments with ///).

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[gaaclarke]

I think these things are making it to stable. Unfortunately it looks like there is a bug in vulkan implementation though (flutter/flutter#180595). So it's probably not worth sending people to this new API quite yet.

[Piinks]

Greetings from stale PR triage! 👋
Is this change still on your radar?

[gaaclarke]

The feature these changes show is not 100% working on stable so we don't want to update the samples yet. They work on main, we're just waiting for the next stable release after 3.41.