[skottie] Fix sphere lighting shader

Clamp s_base to avoid negative pow arguments.

Fixes rendering issues on Mac/Win.

Change-Id: I5176ffa6150ec430c2ab72115b74fd7ede5f5934
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/369482
Commit-Queue: Florin Malita <fmalita@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Florin Malita <fmalita@chromium.org>
Reviewed-by: Brian Osman <brianosman@google.com>

modules/skottie/src/effects/SphereEffect.cpp

@@ -112,7 +112,7 @@ static constexpr char gFancyLightSkSL[] = R"(
 
     half4 apply_light(half3 EYE, half3 N, half4 c) {
         half3 LR = reflect(-l_vec*side_select, N);
-        half s_base = dot(normalize(EYE), LR),
+        half s_base = max(dot(normalize(EYE), LR), 0),
 
         a = l_coeff_ambient,
         d = l_coeff_diffuse  * max(dot(l_vec, N), 0),