Implement buffering

To minimize latency (at the cost of jitter), scrcpy always displays a
frame as soon as it available, without waiting.

However, when recording (--record), it still writes the captured
timestamps to the output file, so that the recorded file can be played
correctly without jitter.

Some real-time use cases might benefit from adding a small latency to
compensate for jitter too. For example, few tens of seconds of latency
for live-streaming are not important, but jitter is noticeable.

Therefore, implement a buffering mechanism (disabled by default) to add
a configurable latency delay.

PR #2417 <https://github.com/Genymobile/scrcpy/issues/2417>

app/src/clock.h

@@ -0,0 +1,70 @@
+#ifndef SC_CLOCK_H
+#define SC_CLOCK_H
+
+#include "common.h"
+
+#include <assert.h>
+
+#include "util/tick.h"
+
+#define SC_CLOCK_RANGE 32
+static_assert(!(SC_CLOCK_RANGE & 1), "SC_CLOCK_RANGE must be even");
+
+struct sc_clock_point {
+    sc_tick system;
+    sc_tick stream;
+};
+
+/**
+ * The clock aims to estimate the affine relation between the stream (device)
+ * time and the system time:
+ *
+ *     f(stream) = slope * stream + offset
+ *
+ * To that end, it stores the SC_CLOCK_RANGE last clock points (the timestamps
+ * of a frame expressed both in stream time and system time) in a circular
+ * array.
+ *
+ * To estimate the slope, it splits the last SC_CLOCK_RANGE points into two
+ * sets of SC_CLOCK_RANGE/2 points, and compute their centroid ("average
+ * point"). The slope of the estimated affine function is that of the line
+ * passing through these two points.
+ *
+ * To estimate the offset, it computes the centroid of all the SC_CLOCK_RANGE
+ * points. The resulting affine function passes by this centroid.
+ *
+ * With a circular array, the rolling sums (and average) are quick to compute.
+ * In practice, the estimation is stable and the evolution is smooth.
+ */
+struct sc_clock {
+    // Circular array
+    struct sc_clock_point points[SC_CLOCK_RANGE];
+
+    // Number of points in the array (count <= SC_CLOCK_RANGE)
+    unsigned count;
+
+    // Index of the next point to write
+    unsigned head;
+
+    // Sum of the first count/2 points
+    struct sc_clock_point left_sum;
+
+    // Sum of the last (count+1)/2 points
+    struct sc_clock_point right_sum;
+
+    // Estimated slope and offset
+    // (computed on sc_clock_update(), used by sc_clock_to_system_time())
+    double slope;
+    sc_tick offset;
+};
+
+void
+sc_clock_init(struct sc_clock *clock);
+
+void
+sc_clock_update(struct sc_clock *clock, sc_tick system, sc_tick stream);
+
+sc_tick
+sc_clock_to_system_time(struct sc_clock *clock, sc_tick stream);
+
+#endif