Simplify clock estimation

The slope encodes the drift between the device clock and the computer
clock. Its real value is expected very close to 1.

To estimate it, just assume it is exactly 1.

Since the clock is used to estimate very close points in the future, the
error caused by clock drift is totally negligible, and in practice it is
way lower than the slope estimation error.

Therefore, only estimate the offset.

app/tests/test_clock.c

@@ -1,79 +0,0 @@
-#include "common.h"
-
-#include <assert.h>
-
-#include "clock.h"
-
-void test_small_rolling_sum(void) {
-    struct sc_clock clock;
-    sc_clock_init(&clock);
-
-    assert(clock.count == 0);
-    assert(clock.left_sum.system == 0);
-    assert(clock.left_sum.stream == 0);
-    assert(clock.right_sum.system == 0);
-    assert(clock.right_sum.stream == 0);
-
-    sc_clock_update(&clock, 2, 3);
-    assert(clock.count == 1);
-    assert(clock.left_sum.system == 0);
-    assert(clock.left_sum.stream == 0);
-    assert(clock.right_sum.system == 2);
-    assert(clock.right_sum.stream == 3);
-
-    sc_clock_update(&clock, 10, 20);
-    assert(clock.count == 2);
-    assert(clock.left_sum.system == 2);
-    assert(clock.left_sum.stream == 3);
-    assert(clock.right_sum.system == 10);
-    assert(clock.right_sum.stream == 20);
-
-    sc_clock_update(&clock, 40, 80);
-    assert(clock.count == 3);
-    assert(clock.left_sum.system == 2);
-    assert(clock.left_sum.stream == 3);
-    assert(clock.right_sum.system == 50);
-    assert(clock.right_sum.stream == 100);
-
-    sc_clock_update(&clock, 400, 800);
-    assert(clock.count == 4);
-    assert(clock.left_sum.system == 12);
-    assert(clock.left_sum.stream == 23);
-    assert(clock.right_sum.system == 440);
-    assert(clock.right_sum.stream == 880);
-}
-
-void test_large_rolling_sum(void) {
-    const unsigned half_range = SC_CLOCK_RANGE / 2;
-
-    struct sc_clock clock1;
-    sc_clock_init(&clock1);
-    for (unsigned i = 0; i < 5 * half_range; ++i) {
-        sc_clock_update(&clock1, i, 2 * i + 1);
-    }
-
-    struct sc_clock clock2;
-    sc_clock_init(&clock2);
-    for (unsigned i = 3 * half_range; i < 5 * half_range; ++i) {
-        sc_clock_update(&clock2, i, 2 * i + 1);
-    }
-
-    assert(clock1.count == SC_CLOCK_RANGE);
-    assert(clock2.count == SC_CLOCK_RANGE);
-
-    // The values before the last SC_CLOCK_RANGE points in clock1 should have
-    // no impact
-    assert(clock1.left_sum.system == clock2.left_sum.system);
-    assert(clock1.left_sum.stream == clock2.left_sum.stream);
-    assert(clock1.right_sum.system == clock2.right_sum.system);
-    assert(clock1.right_sum.stream == clock2.right_sum.stream);
-}
-
-int main(int argc, char *argv[]) {
-    (void) argc;
-    (void) argv;
-
-    test_small_rolling_sum();
-    test_large_rolling_sum();
-    return 0;
-};