RectanizerSkyline: optimize reset

reset() calls fSkyline.reset() and then immediately calls append.
This frees fSkyline's memory and reallocates it again. Change reset
to keep using the already allocated memory, and just reset the size.

Change-Id: I15734f913225a3bccf23bd398613b3cdd6c0c03c
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/587545
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Herb Derby <herb@google.com>
1 file changed