EVENTRECORDER: Add screen update tracking

Screen update boundaries are now used as sync points.
Screenshots are now processed on a screen update boundary.

This change increments the version of the Event Recorder
file format to 2; version 1 files will still play back as
before, without synchronising to screen updates.

backends/modular-backend.cpp

@@ -186,6 +186,8 @@ void ModularGraphicsBackend::fillScreen(uint32 col) {
 
 void ModularGraphicsBackend::updateScreen() {
 #ifdef ENABLE_EVENTRECORDER
+	g_system->getMillis();		// force event recorder to update the tick count
+	g_eventRec.processScreenUpdate();
 	g_eventRec.preDrawOverlayGui();
 #endif