Renamed SpriteMaterial properties: affectedByDistance => sizeAttenuation, mergeWith3D => depthTest. Added support for SpriteMaterial.depthWrite.

SpriteMaterial should be now more consistent with other materials.

See discussion in https://github.com/alteredq/three.js/commit/73922dc797557132a36de1418c78f755185a3a53

I guess before we were not getting errors because it woudln't iterate `undefined`. Now we need to check first.

The result for attributes.size.value[i] can be negative which in turn makes gl_PointSize negative. As an example if Math.sin(...) returns -1 then you'll get -6 as a result. The specification specifically states:"If the value written to gl_PointSize
is less than or equal to zero, results are undefined". In fact it is causing such a problem on our platform.
 

The result for attributes.size.value[i] can be negative which in turn makes gl_PointSize negative. As an example if Math.sin(...) returns -1 then you'll get -6 as a result. The specification specifically states:"If the value written to gl_PointSize
is less than or equal to zero, results are undefined". In fact it is causing such a problem on our platform.
 

Updated EventTarget.dispatchEvent so that it will not iterate over custom members of the Array class

Changed indices to use Uint16Array rather than Int16Array to make full use of the 16bit precision of the pointer

Its sometimes easier to run node.js than python on some computers (eg. on a fresh installation of windows). Although previously discussed in #534, I thought it'd be nice to have a simple web server in jss since a python-less build system is already in place.

This could be an alternative to `python -m SimpleHTTPServer`

To use, cd to the base directory and run
`node utils/simplehttpserver.js <port number>`

Fixes #2637

See #2638

Changed WebGLRenderer's sortFacesByMaterial to only split geometry if MeshFaceMaterial is used at the first render.

Fixes #2638

Before attributes were enabled just once, when creating a new program and then they were never disabled (with exception of UVs).

Now before rendering of the object existing enabled attributes are disabled and attributes required by program are enabled (these GL state changes are cached so they happen only when active geometry or material changes inside a frame).

There is still some gray zone for more exotic rendering methods (immediate render objects and render plugins do not disable outstanding enabled attributes, they just enable attributes they need) but we'll address these if they start to make problems.

Performance seems to be ok, at least for current examples, though there may be some use cases which could get dramatically slower with these changes (before optimizations naive attribute disabling / enabling every time halved the framerate in performance stress test).

Fixes #2633 (hope so)