Expose premultipliedAlpha on material component

[vincentfretin]

Summary

Since three.js r178 (mrdoob/three.js#31246), THREE.MultiplyBlending emits a repeated console warning unless the material has premultipliedAlpha = true:

THREE.WebGLState: MultiplyBlending requires material.premultipliedAlpha = true

A-Frame's material component exposes blending: multiply but does not expose premultipliedAlpha, so users had no way to silence the warning from markup alone.

This PR:

• Adds premultipliedAlpha to the material schema (default false), giving users direct control from markup.
• Forces premultipliedAlpha = true whenever blending is multiply, regardless of the user-supplied value, so the three.js warning cannot accidentally be triggered through the component's public API.

Changes

• src/components/material.js — add premultipliedAlpha schema property; force it on for blending: multiply, otherwise pass through the user value.
• docs/components/material.md — document the new property.
• tests/components/material.test.js — four tests in the blending suite covering:
  • forced on when blending is multiply
  • forced on even when user sets it to false with multiply
  • defaults to false for non-multiply blending
  • respects user value for non-multiply blending

Test plan

• npm test — all tests pass.
• Manually verified the warning no longer appears on an entity using material="blending: multiply; transparent: true".

[dmarcos]

Thank you!