Cache heights-for-range-of-widths instead of height for every width.

Cache heights-for-range-of-widths instead of height for every width.

This patch optimizes window resizes by assuming that if a widget
has the same height at a width of 50 as with a width of 150, the
height for width 100 will also be the same.

The patch also further optimizes the cache allocator, now there
are 2 pointer arrays of up to a maximum of 5 requests, the arrays
will only be allocated if a request is ever made in that orientation
and the array will be sparse until each request is made (i.e. if a
label can only wrap to 3 lines, there will only be 3 out of a
possible 5 SizeRequest structures allocated to cache it).