Make BufferGeometry only contain normal buffer attributes by default

[Methuselah96]

Why

Fixes #389.

#314 added support GLBufferAttribute which made it harder to work with attributes since a lot of three.js code doesn't consider whether an attribute could be a GLBufferAttribute. This PR reduces that friction making it so that geometries don't support GLBufferAttribute by default, and you have to opt-in to them using a generic.

What

Add an Attributes generic to BufferGeometry that specifies what attributes are allowed. This is set to NormalBufferAttributes by default, which does not allow GLBufferAttribute. The consumer can opt-in to allowing GLBufferAttributes by using the NormalOrGLBufferAttributes type or use their own type. This should also pave the way for something like #241 in the future.

I only made it so that Points can handle a geometry with GLBufferAttributes for now. Presumably other objects with geometries will need to allow GLBufferAttributes, but I'm not sure how far to spread it right now. Hopefully we can get can feedback about this from people who actually use GLBufferAttributes once this is released.

Checklist

• Checked the target branch (current goes master, next goes dev)
• Added myself to contributors table
• Ready to be merged