player: use fixed timeout for cache pausing (buffering) duration

This tried to be clever by waiting for a longer time each time the
buffer was underrunning, or shorter if it was getting better. I think
this was pretty weird behavior and makes no sense. If the user really
wants the stream to buffer longer, he/she/it can just pause the player
(the network caches will continue to be filled until they're full).
Every time I actually noticed this code triggering in my own use, I
didn't find it helpful. Apart from that it was pretty hard to test.

Some waiting is needed to avoid that the player just plays the available
data as fast as possible (to compensate for late frames and underrunning
audio). Just use a fixed wait time, which can now be controlled by the
new --cache-pause-wait option.

options/options.c

@@ -451,6 +451,7 @@ const m_option_t mp_opts[] = {
     OPT_FLAG("demuxer-thread", demuxer_thread, 0),
     OPT_FLAG("prefetch-playlist", prefetch_open, 0),
     OPT_FLAG("cache-pause", cache_pausing, 0),
+    OPT_FLOAT("cache-pause-wait", cache_pause_wait, M_OPT_MIN, .min = 0),
 
     OPT_DOUBLE("mf-fps", mf_fps, 0),
     OPT_STRING("mf-type", mf_type, 0),
@@ -894,6 +895,7 @@ const struct MPOpts mp_default_opts = {
     .demuxer_thread = 1,
     .hls_bitrate = INT_MAX,
     .cache_pausing = 1,
+    .cache_pause_wait = 1.0,
     .chapterrange = {-1, -1},
     .ab_loop = {MP_NOPTS_VALUE, MP_NOPTS_VALUE},
     .edition_id = -1,