Consolidate more shadow generation code.
Consolidate more shadow generation code.

Moves the spot shadow code into the base class, and has both shadow
types use it to generate their geometry.

Bug: skia:7971
Change-Id: I866aa8a8a796a5f4bbf4698e1df5148c547ee3c2
Reviewed-on: https://skia-review.googlesource.com/152662
Commit-Queue: Jim Van Verth <509afb0bfbb23ecaaf1fb3323209f531b45e75a6@google.com>
Reviewed-by: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>

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