Use number of indices instead of vertices when setting draw range

[jure]

Description

The GPGPU examples with GLTF birds uses vertices per bird as a way to set the draw range. Since this is indexed geometry this results in a rather sad looking flamingo (in fact even if the count is set to 2, it barely draws one bird):

Using the number of indices instead fixes this. Look at this pretty bird:

A side result is that there are now a lot more birds, before we were simulating them all but drawing only a part of their flock. :)

[Mugen87]

https://raw.githack.com/jure/three.js/fix_gpgpu_birds_draw_range/examples/webgl_gpgpu_birds_gltf.html

A side result is that there are now a lot more birds

Indeed, the example looks now a bit crowded. It's probably best to lower number of maximum birds from 4096 to 1024.

[jure]

I've set the default number of birds to be 1/4th of the total number of birds, but still keep that maximum of 4096 as an option in the controls. The sizes of those textures seem appropriate and perform well and simulating 4096 of something shows the capabilities well. Does that sound reasonable?

[mrdoob]

Thanks!