GrTessellator: eliminate some redundant computation.
GrTessellator: eliminate some redundant computation.

No need to recompute the bisector when mitering.

Change-Id: If7b99244d530d4b2241b7b45611f9401b2f02d69
Reviewed-on: https://skia-review.googlesource.com/93673
Reviewed-by: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>
Commit-Queue: Stephen White <30b0ee0ef1651e8ba59cfd41f16e23abfe9aca8a@chromium.org>

aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,HalCanary/skia-hc,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,rubenvb/skia,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,google/skia,rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,google/skia