Optimize KE shading ("200 atoms with KE shading": 38fps -> 51fps!)

[#57655302]

Note that I get 51fps also for the same test without KE shading. So
now we get KE shading for free.

This commits ensure that we end up only with only two WebGL batches
(one for base atom sprites, second for KE shading sprites). Previously
we had 400 batches, each contained just one atom or KE sprite.
In general PixiJS should handle that, but...

See comments here: https://www.pivotaltracker.com/story/show/57655302
or Pixi issue: https://github.com/GoodBoyDigital/pixi.js/issues/351

Note that we have to ensure that sprites using the same texture are
created together, e.g.:
new PIXI.Sprite(tex1);
new PIXI.Sprite(tex1);
new PIXI.Sprite(tex2);
new PIXI.Sprite(tex2);
...

instead of:

new PIXI.Sprite(tex1);
new PIXI.Sprite(tex2);
new PIXI.Sprite(tex1);
new PIXI.Sprite(tex2);
...

Also, when I add keSprite to atom (using .addChild()) it causes that
I end up with way too many WebGL batches.