Change InstancedMesh api signature to count, geom, mat

Description of the problem

The current signature is unfortunate, since geom and mat have defaults (which means they should be last), but count has to be supplied (it should therefore be first). This causes us some headaches when we need to create an instancedmesh declaratively, like so:

<instancedMesh args={[null, null, count]}>
  <sphereBufferGeometry attach="geometry />
  <meshBasicMaterial attach="material" />

declarative use or not, this is also weird in the imperative way:

const m = new THREE.InstancedMesh(null, null, count)
m.geometry = ...
m.material = ...

the proposal would be:

new THREE.InstancedMesh(count)
new THREE.InstancedMesh(count, geom)
new THREE.InstancedMesh(count, geom, mat)

which i think is natural, in js and ts alike

(count: number, geom?: THREE.Geometry, mat?: THREE.Material)

examples:

https://codesandbox.io/s/three-fiber-useloader-pjcc1

https://codesandbox.io/s/react-three-fiber-gestures-xj31x?from-embed

The prototype is still very new and hasn’t even been documented. Can we fix this before it’s too late? @mrdoob

[donmccurdy]

@drcmda for context, many threejs objects (Mesh, Line, LineLoop, LineSegments, Points, SkinnedMesh) share geometry, material as the first two arguments. While they aren’t technically required arguments, there are exceedingly few cases where the user would not provide them in the constructor. I think we’re hoping for a solution that maintains consistency with those method signatures.

[drcmda]

i think defaults coming last and required vars first is a standard? in typescript for instance:

but this is exactly what instancedmesh does, two optionals, followed by a required one. these little dings and dongs are just more visible when you put them into a declarative format.