add index in getPixels for F16

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1888883003

Review URL: https://codereview.chromium.org/1888883003
diff --git a/src/core/SkLinearBitmapPipeline_sample.h b/src/core/SkLinearBitmapPipeline_sample.h
index d436e39..539ce00 100644
--- a/src/core/SkLinearBitmapPipeline_sample.h
+++ b/src/core/SkLinearBitmapPipeline_sample.h
@@ -777,7 +777,7 @@
     }
 
     Sk4f getPixelAt(const void* vsrc, int index) {
-        const uint64_t* src = static_cast<const uint64_t*>(vsrc);
+        const uint64_t* src = static_cast<const uint64_t*>(vsrc) + index;
         return SkHalfToFloat_01(*src);
     }