Add picking to RenderCore.

*  ui5/eve7/lib/GlViewerRCore.js
   - Picking for RCore.
   - Somewhat better RCore bootstrap, need real module loader with RCore+Reve stuff.
   - Enable EXT_color_buffer_float.

* ui5/eve7/lib/RendeQuTor.js
  - Add picking render pass, should really use reduced image and
    small region around camera.
  - Depth extraction -- partial.

* ui5/eve7/lib/EveElementsRCore.js
  Picking title.

* ui5/eve7/shaders/programs.json
  Register copyDepth2RReve shaders.

* ui5/eve7/shaders/custom/copyDepth2RReve.frag
* ui5/eve7/shaders/custom/copyDepth2RReve.vert
  New files: Copy depth buffer to r32f texture.