WebGLRenderer: Fix issue 19430. and enable rendering without vertex data.

[sjpt]

Allow use of procedural geometry (a) with no index or position attributes, and/or (b) no instance attributes.

[sjpt]

Note: some discussion on background at #19430
This also resolves the original issue raised there.

[sjpt]

Original sample at https://jsfiddle.net/sjpt/Lrhjkstz
For a more realistic example of procedural geometry see https://jsfiddle.net/sjpt/hgfsvLzn

[xinaesthete]

hacky replacement of gui with dat.gui version for second sample above https://jsfiddle.net/kse6vd3g/

[sjpt]

Changed title to reflect that this resolves the change introduced in 117 that broke code that previously worked, as well as allowing new usage.

[duhaime]

2**64 thanks @sjpt! I just hit this exact problem attempting to upgrade from 110 to 111 and had a heck of a time figuring out what was going on.

It turns out I was not passing an attribute named "position" to my BufferGeometry. I was passing positional information, but under a different buffer name.

In version 110, this was allowed, and I could just set my vertex position in the vertex shader without issue. In version 111, nothing is rendered unless I rename my buffer with position data "position"!

[filonik]

I just submitted an issue (#24165) that got rightfully closed as a duplicate. This looks like a great improvement. Any chance of something like this being adopted? What is still lacking? It is a bit discouraging that the last activity seems 2 years ago...

Edit: I suppose one thing that could be improved is cleaning up logic/control flow. I think it is a bit hard to follow. Essentially, the problem seems to be that we want to compute drawStart and drawEnd, while being fairly defensive about it to prevent indexing outside of the provided buffers. I am not sure if it is necessary to be that defensive, but that's a different story. Regardless, in my mind, the logic should simply go as follows (prioritized in the given order):

1. Initialize drawStart and drawEnd with the values in drawRange.
2. If a group is specified, update drawStart and drawEnd accordingly.
3. If the geometry is indexed, clamp drawStart and drawEnd to 0 and index.count (optional).
4. If the geometry is non-indexed with a position attribute, clamp drawStart and drawEnd to 0 and position.count (optional).

I have labelled the last two as optional, because personally I would generally prefer a warning rather than silently changing the given drawRange under the hood. But I understand if it is not desirable to break the existing behavior.

[Mugen87]

Closing in favor of #24179.