GrTessellator (AA): Performance tweaks and cleanup.

The SkArenaAlloc destructor was showing up as hot in profiling,
especially on Linux.  The reason is that it was being used
incorrectly: the size estimate was being used as the chunk size. It
turns out that the best performance seems to be achieved with no
initial allocations and a fixed chunk size of 16K, as the CPU path
renderer does.

Also, allocate the bisectors used for edge inversions on the
stack, not the heap. And remove some unused code.

BUG=skia:

Change-Id: I754531c753c9e602713bf2c8bb5a0eaf174bb962
Reviewed-on: https://skia-review.googlesource.com/8560
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
1 file changed