[core] TransitionOptions s/operator bool/isDefined/

include/mbgl/style/transition_options.hpp

@@ -18,7 +18,7 @@ public:
         };
     }
 
-    explicit operator bool() const {
+    bool isDefined() const {
         return duration || delay;
     }
 };

src/mbgl/style/paint_property.hpp

@@ -35,7 +35,7 @@ public:
         : begin(now + transition.delay.value_or(Duration::zero())),
           end(begin + transition.duration.value_or(Duration::zero())),
           value(std::move(value_)) {
-        if (transition) {
+        if (transition.isDefined()) {
             prior = { std::move(prior_) };
         }
     }