Improve XPath allocator performance

When allocating new pages, make sure that the page has at least 1/4 of the
base page size free. This makes sure that we can do small allocations after
big allocations (i.e. huge node lists) without doing a heap alloc.

This is important because XPath stack code always reclaims extra pages after
evaluating sub-expressions, so allocating a small chunk of memory and then
rolling the state back is a common case (filtering a node list using a
predicate usually does this).

A better solution involves smarter allocation rollback strategy, but the
implemented solution is simple and practical.

git-svn-id: https://pugixml.googlecode.com/svn/trunk@999 99668b35-9821-0410-8761-19e4c4f06640