Fix Processing_api.h: restore header comment, add deltaTime forwarder

Author: pepc84

src/Processing_api.h

@@ -1,4 +1,6 @@
-// Processing_api.h
+/
+inline float deltaTime()   { return ::Processing::PApplet::g_papplet ? ::Processing::PApplet::g_papplet->deltaTime : 0.016f; }
+/Processing_api.h
 // Included in Sketch_run.cpp to make Processing API available to user-defined
 // classes inside sketches (which don't inherit from PApplet).
 #pragma once