GrTessellator (AA): sorting and comparison performance improvements.

GrTessellator (AA): sorting and comparison performance improvements.

Change comparison functions to perform the common case first (a.fX > b.fX),
and the uncommon case (a.fX == b.fX) after the short-circuit.

Change Comparator to switch on a direction enum instead of using function
pointers.

Inline sorted_merge() and front_back_split() into merge_sort(), and template
it on the comparator function, so it instantiates two versions. This
is even faster (but costs us some code bloat of course).

BUG=skia:

Change-Id: I45a2376492240ed7e0552ca2aed75e303e918bc6
Reviewed-on: https://skia-review.googlesource.com/8791
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>