Introduce into Points, per-Point (vertex) alpha

[vveisard]

Describe the feature you'd like:

three's default points supports per-vertex opacity: https://codesandbox.io/s/react-three-fiber-per-vertex-opacity-6ox99v

I would drei's Points API (buffer and Point) to support individual opacity, for feature parity.

Suggested implementation:

To keep drei's API congruent with three, I would suggest allowing passing of a 4-tuple to Point's color prop, like so: https://codesandbox.io/s/react-three-drei-per-vertex-opacity-p6by7r?file=/src/App.tsx

This alone is not enough, because Point's color attribute only supports a 3-tuple, and itemSize attribute is not exposed. I would suggest that, when the transparent prop is present on Points, to set itemSize 4 for color. This is a bit "magical", but it is straightforward and expected.

[benrugg]

I would also like to see this feature! I'm not sure how best to implement it, but it's definitely an important effect in some situations to be able to control per-point opacity. (For variation and for fading in and out!)