[skotty] Add cubic Bezier lerp stubs

... and refactor some of the keyframe parsing.

TBR=
Change-Id: If45922eab36412908036401cee693202f5c3e281
Reviewed-on: https://skia-review.googlesource.com/91100
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/experimental/skotty/SkottyAnimator.cpp b/experimental/skotty/SkottyAnimator.cpp
index 451f146..d4c5c96 100644
--- a/experimental/skotty/SkottyAnimator.cpp
+++ b/experimental/skotty/SkottyAnimator.cpp
@@ -42,4 +42,16 @@
     SkAssertResult(fV1.fPath.interpolate(fV0.fPath, t, &v->fPath));
 }
 
+float AnimatorBase::ComputeLocalT(float t, float t0, float t1,
+                                  const SkPoint& c0, const SkPoint& c1) {
+    SkASSERT(t1 > t0);
+    auto lt = (t - t0) / (t1 - t0);
+
+    if (c0 != SkPoint({0, 0}) || c1 != SkPoint({1, 1})) {
+        // TODO: lt = CubicBezier(lt, c0, c1);
+    }
+
+    return SkTPin<float>(lt, 0, 1);
+}
+
 } // namespace skotty