vo_opengl: fix out-of-bounds access in timer_pool_measure

This was there even before the refactor, but the refactor exposed the
bug. I hate C's useless fucking modulo operator so much. I've gotten hit
by this exact bug way too many times.

video/out/opengl/utils.c

@@ -293,8 +293,9 @@ struct mp_pass_perf timer_pool_measure(struct timer_pool *pool)
     if (!pool)
         return (struct mp_pass_perf){0};
 
+    int last = (pool->sample_idx ? pool->sample_idx : VO_PERF_SAMPLE_COUNT) - 1;
     struct mp_pass_perf res = {
-        .last = pool->samples[(pool->sample_idx - 1) % VO_PERF_SAMPLE_COUNT],
+        .last = pool->samples[last],
         .avg = pool->sample_count > 0 ? pool->sum / pool->sample_count : 0,
         .peak = pool->peak,
         .count = pool->sample_count,