Don't switch kDecal to kClamp

Came across this when working on a new GM that happened to use a 1px
wide image with decal sampling, this caused it to smear across a much
larger than expected area.

Change-Id: I05a12e4ae9a2a54f8a9a18050573a4f6b35197ac
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/451357
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>

src/shaders/SkImageShader.cpp

@@ -60,7 +60,9 @@ static SkTileMode optimize(SkTileMode tm, int dimension) {
     // for transforming to clamp.
     return tm;
 #else
-    return dimension == 1 ? SkTileMode::kClamp : tm;
+    // mirror and repeat on a 1px axis are the same as clamping, but decal will still transition to
+    // transparent black.
+    return (tm != SkTileMode::kDecal && dimension == 1) ? SkTileMode::kClamp : tm;
 #endif
 }