make float more primary API for paint (esp. for alpha)

Bug: skia:8773
Change-Id: I82b1f22f300eadc93f79a35a1638b7eb6376169a
Reviewed-on: https://skia-review.googlesource.com/c/193031
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
diff --git a/gm/shapes.cpp b/gm/shapes.cpp
index 0b01390..63ddf2a 100644
--- a/gm/shapes.cpp
+++ b/gm/shapes.cpp
@@ -95,7 +95,7 @@
         for (int i = 0; i < fShapes.count(); i++) {
             SkPaint paint(fPaint);
             paint.setColor(rand.nextU() & ~0x808080);
-            paint.setAlpha(128);  // Use alpha to detect double blends.
+            paint.setAlphaf(0.5f);  // Use alpha to detect double blends.
             const SkRRect& shape = fShapes[i];
             canvas->save();
             canvas->rotate(fRotations[i]);
@@ -141,7 +141,7 @@
             inner.transform(innerXform, &xformedInner);
             SkPaint paint(fPaint);
             paint.setColor(rand.nextU() & ~0x808080);
-            paint.setAlpha(128);  // Use alpha to detect double blends.
+            paint.setAlphaf(0.5f);  // Use alpha to detect double blends.
             canvas->save();
             canvas->rotate(fRotations[i]);
             canvas->drawDRRect(outer, xformedInner, paint);