Fix: framerate window showed a slightly higher rate than actually measured (#8682)

The first point was counted, but also initialized as "last". As
such, it didn't add to "total", but did add to "count", which made
the "count" 1 more than the total actually represents.

src/framerate_gui.cpp

@@ -151,6 +151,10 @@ namespace {
 			/* Total duration covered by collected points */
 			TimingMeasurement total = 0;
 
+			/* We have nothing to compare the first point against */
+			point--;
+			if (point < 0) point = NUM_FRAMERATE_POINTS - 1;
+
 			while (point != last_point) {
 				/* Only record valid data points, but pretend the gaps in measurements aren't there */
 				if (this->durations[point] != INVALID_DURATION) {