Hopefully no more deprecation warnings.

Cleaned up some more examples.

Problem was that objects could be added to WebGLRenderer's internal render list only once (because of `webglInit` flag).

Now there are two separate internal states (`webglInit` and `webglActive`) so that object can be initialized just once (expensive operation) but added / removed from the scene many times (cheap operation).

Also removed unused material Sprite parameter.

This buggy pattern ("something = parameter || nonzero value") exists at more places though elsewhere setting to zero doesn't make that much sense (e.g. in geometries).

Not sure if we should change it everywhere to proper pattern ("something = ( parameter !== undefined ) ? parameter : value").

Maybe it would be good for establishing habit. I remember fixing this buggy pattern long ago but it just keeps popping up.

Added renderer.getClearAlpha. Extended RenderPass to be able to use custom alpha clear. Fixed UVs in UTF8Loader.

Fixed GeometryUtils.merge (normals and centroids weren't being multiplied)
Fixed misc_lookat and misc_ubiquity_test examples.

This allows, for example, better integration of additively blended materials with a regular scene.

Not to be confused with depthTest (which controls complete access to depth buffer, including reading).

Also messed up a bit with particle emitter example.

Simplified `Trident`.
Added `setX`, `setY`, `setZ` methods to `Vector3`. They're handy for oneliners (see `CylinderGeometry`).