Fix compilation with the latest version of nCine

- Rename the interval method of the application class
- Add missing cmath header to the easing curve class implementation

Author: encelo

src/EasingCurve.cpp

@@ -1,3 +1,4 @@
+#include <cmath>
 #include <ncine/common_constants.h>
 #include "EasingCurve.h"
 

src/main.cpp

@@ -179,7 +179,7 @@ void MyEventHandler::onShutdown()
 
 void MyEventHandler::onFrameStart()
 {
-	const float interval = nc::theApplication().interval();
+	const float frameTime = nc::theApplication().frameTime();
 	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
 
 	theCanvas->bind();
@@ -196,7 +196,7 @@ void MyEventHandler::onFrameStart()
 		}
 	}
 	else
-		theAnimMgr->update(interval);
+		theAnimMgr->update(frameTime);
 	theSpriteMgr->update();
 
 	theCanvas->unbind();