vo_opengl: refactor vo performance subsystem

This replaces `vo-performance` by `vo-passes`, bringing with it a number
of changes and improvements:

1. mpv users can now introspect the vo_opengl passes, which is something
   that has been requested multiple times.

2. performance data is now measured per-pass, which helps both
   development and debugging.

3. since adding more passes is cheap, we can now report information for
   more passes (e.g. the blit pass, and the osd pass). Note: we also
   switch to nanosecond scale, to be able to measure these passes
   better.

4. `--user-shaders` authors can now describe their own passes, helping
   users both identify which user shaders are active at any given time
   as well as helping shader authors identify performance issues.

5. the timing data per pass is now exported as a full list of samples,
   so projects like Argon-/mpv-stats can immediately read out all of the
   samples and render a graph without having to manually poll this
   option constantly.

Due to gl_timer's design being complicated (directly reading performance
data would block, so we delay the actual read-back until the next _start
command), it's vital not to conflate different passes that might be
doing different things from one frame to another. To accomplish this,
the actual timers are stored as part of the gl_shader_cache's sc_entry,
which makes them unique for that exact shader.

Starting and stopping the time measurement is easy to unify with the
gl_sc architecture, because the existing API already relies on a
"generate, render, reset" flow, so we can just put timer_start and
timer_stop in sc_generate and sc_reset, respectively.

The ugliest thing about this code is that due to the need to keep pass
information relatively stable in between frames, we need to distinguish
between "new" and "redrawn" frames, which bloats the code somewhat and
also feels hacky and vo_opengl-specific. (But then again, this entire
thing is vo_opengl-specific)

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