Implement four more xfermodes with Sk4px.

src/core/Sk4px.h

Index: src/core/Sk4px.h
diff --git a/src/core/Sk4px.h b/src/core/Sk4px.h
index 26d4d0f865281c19ae30da95143f650f231d4e9d..e046e265fee1fad30d141ba8e96cae61d0a5f0f0 100644
--- a/src/core/Sk4px.h
+++ b/src/core/Sk4px.h
@@ -85,6 +85,8 @@ public:
     // These just keep the types as Sk4px so the user doesn't have to keep casting.
     Sk4px operator + (const Sk4px& o) const { return INHERITED::operator+(o); }
     Sk4px operator - (const Sk4px& o) const { return INHERITED::operator-(o); }
+    Sk4px operator < (const Sk4px& o) const { return INHERITED::operator<(o); }
+    Sk4px thenElse(const Sk4px& t, const Sk4px& e) const { return INHERITED::thenElse(t,e); }
 
     // Generally faster than (*this * o).div255().
     // May be incorrect by +-1, but is always exactly correct when *this or o is 0 or 255.