Fix zigzag encoding overflow

[anandthakker]

The current bit-shifting arithmetic for zigzag encoding:

vt-pbf/index.js

Lines 125 to 127 in ad933a6

	function zigzag (num) {
	return (num << 1) ^ (num >> 31)
	}

overflows for 2^31 - 1, because JavaScript bit-shifting yields signed 32-bit integers: https://www.ecma-international.org/ecma-262/5.1/#sec-11.7.1

This is unlikely to matter in practice, given that tile extents are typically 4096 or 8192