This is an index into an array of items (points or polygons), or a collection of items
each uniquely identified by an integer (clips or envelopes). A VX is written as a variable
length 2- or 4-byte element. If the index value is less than 65,280 (0xFF00), then the
index is written as an unsigned two-byte integer. Otherwise the index is written as an
unsigned four byte integer with bits 24-31 set. When reading an index, if the first byte
encountered is 255 (0xFF), then the four-byte form is being used and the first byte should
be discarded or masked out.