Remove SegmentVector abstraction

Nobody has claimed this yet.

We introduced `SegmentVector` class to be able to render layers with >2^16 vertices — by maintaining multiple views ("segments") in the vertex array with different offsets. However, according to [WebGL best practices](https://developer.mozilla.org/en-US/docs/Web/API/WebGL_API/WebGL_best_practices), `OES_element_index_uint` extension that allows 32-bit vertex indexing as opposed to 16-bit one is universally supported and can be relied upon. We could use it to remove the segments abstraction, using the extension to render >2^16 vertices with a single draw draw call, and removing the complexity around splitting and maintaining segments.

Start by locating the SegmentVector class and the code that maintains segment offsets, then inspect the WebGL index-buffer path and OES_element_index_uint support. Done means the segment abstraction and its maintenance complexity are removed while layers with more than 2^16 vertices render through a single draw call.

Written by the indexing model from the issue text.