[DisplayList] Use internal conversion for opacity->alpha (#165349)

Removing an unnecessary dependence on an external library for doing
opacity->alpha conversions in DlPaint.

engine/src/flutter/display_list/dl_paint.h

@@ -76,8 +76,7 @@ class DlPaint {
   DlPaint& setAlpha(uint8_t alpha) { return setColor(color_.withAlpha(alpha)); }
   DlScalar getOpacity() const { return color_.getAlphaF(); }
   DlPaint& setOpacity(DlScalar opacity) {
-    setAlpha(SkScalarRoundToInt(opacity * 0xff));
-    return *this;
+    return setAlpha(DlColor::toAlpha(opacity));
   }
 
   DlBlendMode getBlendMode() const {