Support multiple Buffers sources for Attributes (both interleaved and packed)

[yanchith]

Currently, we can only pass in one buffer per attribute, but it is often desired to have multiple data sources in a single buffer, e.g. by interleaving values [position, color, position, color, ...].

WebGL let's the user control this via stride and offset parameters to gl.vertexAttribPointer(). Since we already expose other parameters to gl.vertexAttribPointer(), we can expose these two as well.

Also note that currently it is possible to pass a number[] as buffer in AttributePointerConfig. This must be disallowed since it would silently upload the same vertex buffer multiple times. This is no longer possible to do, so we should be good.