Issue #1925 - Convert NS_FULL_TO_HALF_CORNER to a constexpr function.

Note: Because the new function checks types, we need to change the
fullCorner type in `nsComputedDOMStyle::GetEllipseRadii()` and
`StyleAnimationValue::ExtractComputedValue()` to `Corner` instead of the
underlying base type.

gfx/2d/Types.h

@@ -469,6 +469,11 @@ constexpr Corner HalfToFullCorner(HalfCorner aHalfCorner)
   return Corner(aHalfCorner / 2);
 }
 
+constexpr HalfCorner FullToHalfCorner(Corner aCorner, bool aIsVertical)
+{
+  return HalfCorner(aCorner * 2 + aIsVertical);
+}
+
 } // namespace mozilla
 
 #endif /* MOZILLA_GFX_TYPES_H_ */