Function isSharedStageMaterial

isSharedStageMaterial(material: Material): material is ISharedStageMaterial
Type guard function to check if a material uses SharedStageTexture.

This utility function helps ensure type safety when working with materials that should contain a SharedStageTexture as their map property.
Parameters
- material: Material
  The Three.js material to check
Returns material is ISharedStageMaterial
True if the material has a SharedStageTexture map, false otherwise
Example
```
if (isSharedStageMaterial(material)) {
  // TypeScript now knows material.map is SharedStageTexture
  material.map.setNeedUpdate();
  const uv = material.map.calculateUV(textureArea);
}
```
- Defined in src/SharedStageTexture.ts:293