av_info: report the actual SPG-derived refresh rate

retro_get_system_av_info bucketed timing.fps to 60.00 (VGA) / 59.94 (NTSC) /
50.00 (PAL) from a pixel-clock table whose constants didn't even reconcile with
those figures, and the VGA case was wrong: Dreamcast VGA runs at 59.94, not 60
(27MHz / (858*525) = 59.9401). It also missed that 240p NTSC is ~59.83, not
59.94, and NAOMI boards run faster still.

The scheduler already computes the exact frame period in CalculateSync from the
SPG registers the game programs (Line_Cycles * pvr_numscanlines). Expose it as
spg_get_refresh_rate() -- pixel_clock / ((hcount+1) * (vcount+1)) -- and report
that. timing.fps now equals the rate frames and audio are actually produced at,
per mode and per game: 59.94 for VGA/480i, ~59.83 for 240p NTSC, 50.00 for PAL
480i, ~49.92 for PAL 240p, and the correct higher rates for NAOMI. Matches the
hardware-derived values confirmed upstream (flyinghead/flycast PR #2033) and
keeps frontend frame pacing in step with the core, pairing with the per-frame
audio flush. Falls back to 60 before CalculateSync has run.

core/hw/pvr/spg.cpp

@@ -59,6 +59,24 @@ void CalculateSync(void)
 	sh4_sched_request(vblank_schid, Line_Cycles);
 }
 
+/*
+ * Actual emulated vertical refresh rate, derived from the same SPG timing the
+ * scheduler runs on: pixel_clock / ((hcount+1) * (vcount+1)), i.e.
+ * SH4_MAIN_CLOCK / (Line_Cycles * pvr_numscanlines). This is the rate at which
+ * frames (and thus audio) are actually produced, so reporting it to the
+ * frontend via retro_get_system_av_info keeps timing.fps in step with the core
+ * instead of a bucketed 60/59.94/50 guess. It naturally yields the hardware
+ * figures: ~59.94 for VGA and 480i, ~59.83 for 240p NTSC, 50.00 for PAL 480i,
+ * and the higher NAOMI rates for boards that program the SPG accordingly.
+ * Falls back to 60 before CalculateSync has run.
+ */
+double spg_get_refresh_rate(void)
+{
+	if (Line_Cycles == 0 || pvr_numscanlines == 0)
+		return 60.0;
+	return (double)SH4_MAIN_CLOCK / ((double)Line_Cycles * (double)pvr_numscanlines);
+}
+
 
 //called from sh4 context , should update pvr/ta state and everything else
 int spg_line_sched(int tag, int cycl, int jit)

core/hw/pvr/spg.h

@@ -6,6 +6,7 @@ void spg_Term();
 void spg_Reset(bool Manual);
 
 void CalculateSync();
+double spg_get_refresh_rate();
 void read_lightgun_position(int x, int y);
 struct TA_context;
 void SetREP(TA_context* cntx);

core/libretro/libretro.cpp

@@ -2304,10 +2304,6 @@ void retro_get_system_info(struct retro_system_info *info)
 
 void retro_get_system_av_info(struct retro_system_av_info *info)
 {
-   /*                        00=VGA    01=NTSC   10=PAL,   11=illegal/undocumented */
-   const int spg_clks[4] = { 26944080, 13458568, 13462800, 26944080 };
-   u32 pixel_clock= spg_clks[(SPG_CONTROL.full >> 6) & 3];
-
    if (cheatManager.Reset())
    {
       info->geometry.aspect_ratio = 16.0 / 9.0;
@@ -2336,24 +2332,11 @@ void retro_get_system_av_info(struct retro_system_av_info *info)
    if (rotate_screen)
       info->geometry.aspect_ratio = 1 / info->geometry.aspect_ratio;
 
-   switch (pixel_clock)
-   {
-      case 26944080:
-         info->timing.fps = 60.00; /* (VGA  480 @ 60.00) */
-         break;
-      case 26917135:
-         info->timing.fps = 59.94; /* (NTSC 480 @ 59.94) */
-         break;
-      case 13462800:
-         info->timing.fps = 50.00; /* (PAL 240  @ 50.00) */
-         break;
-      case 13458568:
-         info->timing.fps = 59.94; /* (NTSC 240 @ 59.94) */
-         break;
-      case 25925600:
-         info->timing.fps = 50.00; /* (PAL 480  @ 50.00) */
-         break;
-   }
+   /* Report the actual emulated refresh rate, derived from the SPG timing the
+    * scheduler runs on, so the frontend's frame pacing matches the rate frames
+    * and audio are really produced at (~59.94 VGA/480i, ~59.83 240p NTSC,
+    * 50.00 PAL 480i, higher for NAOMI) rather than a bucketed 60/59.94/50. */
+   info->timing.fps = spg_get_refresh_rate();
 
    info->timing.sample_rate = 44100.0;
 }