player/CrossFade: add method IsEnabled()

src/player/CrossFade.cxx

@@ -96,8 +96,8 @@ CrossFadeSettings::Calculate(SignedSongTime total_time,
 {
 	unsigned int chunks = 0;
 
-	if (total_time.IsNegative() ||
-	    duration <= FloatDuration::zero() ||
+	if (!IsEnabled() ||
+	    total_time.IsNegative() ||
 	    duration >= std::chrono::duration_cast<FloatDuration>(total_time) ||
 	    /* we can't crossfade when the audio formats are different */
 	    af != old_format)

src/player/CrossFade.hxx

@@ -39,6 +39,10 @@ struct CrossFadeSettings {
 	 */
 	FloatDuration mixramp_delay{-1};
 
+	constexpr bool IsEnabled() const noexcept {
+		return duration.count() > 0;
+	}
+
 	/**
 	 * Calculate how many music pipe chunks should be used for crossfading.
 	 *