Screens: frame rates by attention, ticks in step with the display

ft-screens gave KWin a frame callback for every committed screen on each tick, and the tick
was an 11 ms timer set again after each run, so it slid through the display's frame and came
about 85 times a second at 90 Hz: the desktop repeated a frame several times a second (judder
in scrolling and video), and KWin drew every screen in one burst at a random point of
vrcompositor's frame. Hidden screens got the same 90 Hz unless Frametop was paused for a game.

- Ticks run on a timerfd at absolute times, once per display frame, 1 ms after the vsync
  (IVRSystem::GetTimeSinceLastVsync and the HMD's display frequency, read once a second), so
  KWin gets its callbacks early in the frame. Measured with --no-vr: 91 wakeups a second
  instead of about 85. On the Frame the vsync times SteamVR reports lie on a 90 Hz grid.
- Each screen's callbacks come at a rate for how much of it you see (vr.cpp,
  UpdateAttention): every frame while focused (within 12 degrees of where your head points,
  a laser or the mouse on it in the last 1.5 s, carried, or typed on), 15 a second for the
  rest of what you see (within 60 degrees), and 1 a second when hidden, behind you, or
  paused. Levels rise at once and fall after 1.5 s (focused) or 0.5 s (in view). KWin draws
  a screen only after its callback and its apps wait for theirs, so this throttles the apps
  too. A screen where nothing changes costs nothing at any rate, as before.
- A video in view keeps every frame: 8 commits in a row that each redraw 6% or more of the
  screen, at 10 a second or more, count as one (from the surface's buffer damage).
- "rates F V H" / --rates set the three rates (default 0 15 1, 0 = every frame), "rates?"
  shows them and each screen's level, "watch S" gives everything full rate for S seconds for
  a remote viewer (vnc-bridge.sh renews it), and "phase MS" moves the ticks for tuning.
- ft-screens' main thread runs at nice -5 after the session starts: SteamOS allows down to
  -8 once the soft RLIMIT_NICE is raised, and KWin waits on these ticks. It had spent nearly
  3 times as long waiting to run as running.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

screens/vr.h

@@ -38,6 +38,14 @@ int ft_vr_modifiers(uint32_t format, uint64_t *out, int max);
 bool ft_vr_screens_shown(void);
 // Frametop is paused for a VR game ("pause on"): everything is hidden, and KWin slows down.
 bool ft_vr_paused(void);
+// How much of a screen you see, for its frame rate (compositor.c): hidden (or out of view),
+// in view, or focused (you look at it, or a laser or the mouse is on it). Focused without
+// SteamVR (--no-vr) or for an unknown screen.
+enum ft_attention { FT_HIDDEN, FT_IN_VIEW, FT_FOCUSED };
+enum ft_attention ft_vr_screen_attention(int index);
+// The display's refresh rate and the time since its last vsync, in seconds. False without
+// them (no SteamVR, or the headset isn't reporting).
+bool ft_vr_vsync(double *since, double *hz);
 // A panel for screen `index`, width in metres, placed in a row in front of the head.
 void ft_vr_screen_create(int index, double metres, int count);
 void ft_vr_screen_destroy(int index);