Green refactor of Render-to-texture: Use a Texture instance inside WebGLRenderTarget rather than duplicating all internal texture fields.

Point light shadow mapping now utilizes cube-to-2D projection, avoiding the need for a samplerCube in the shader.

We have to bind the buffer before making the test, because the answer is circumstantial ( https://www.khronos.org/registry/webgl/specs/1.0/#readpixels ).

Did you know that with the OES_texture_float extension Chrome allows rendering to a float texture then reading a float render target to a Float32ArrayBuffer in readPixels()  ?

WebGLRenderer: Combined ImmediateRenderObjects and normal objects. See #7160. @tschw also sounds good? :)

* Shorter (less typo-prone)
* More intuitive (also "aligns" with gl.POINTS)
* Makes a nice API (Mesh, Line, Points).