Fix autoswitch not working with 4K monitor @25/30Hz

src/defs.h

@@ -106,7 +106,7 @@
 #define BIT_NO_SKIP_HSYNC         0x08000000  // bit 27, clear if hsync is ignored (used by cache preload)
 #define BIT_HSYNC_EDGE            0x10000000  // bit 28, clear if trailing edge
 #define BIT_RPI234                0x20000000  // bit 29, set if Pi 2, 3 or 4 detected
-//#define BIT_                    0x40000000  // bit 30,
+#define BIT_SKIP_ALT_FRAME        0x40000000  // bit 30, set to skip capture of alternate frames used in 4K@25/30Hz
 //#define BIT_                    0x80000000  // bit 31, may get corrupted - check
 
 // R0 return value bits

src/rgb_to_fb.S

@@ -1271,7 +1271,7 @@ noInBandData:
         ldr    r0, param_video_type
         cmp    r0, #VIDEO_TELETEXT
         beq    skip_osd_update
-        tst    r3, #BIT_OSD
+        tst    r3, #BIT_OSD | BIT_SKIP_ALT_FRAME
         beq    skip_osd_update
 
         push   {r1-r5, r11}

src/rgb_to_hdmi.c

@@ -3480,10 +3480,10 @@ void rgb_to_hdmi_main() {
          int old_flags = flags;
          if (half_frame_rate) {   //half frame rate display detected (4K @ 25Hz / 30Hz)
              if  (capinfo->vsync_type != VSYNC_NONINTERLACED_DEJITTER) {   //inhibit alternate frame dropping when using the vertical dejitter mode as that stops it working
-                 if ((flags & BIT_OSD) == 0) {
-                    capinfo->palette_control |= INHIBIT_PALETTE_DIMMING_16_BIT;   //if OSD not enabled then stop screen dimming when blank OSD turned on below
-                    flags |= BIT_OSD;          //turn on OSD even though it is blank to force dropping of alternate frames to eliminate tear
-                 }
+                 //if ((flags & BIT_OSD) == 0) {
+                 //   capinfo->palette_control |= INHIBIT_PALETTE_DIMMING_16_BIT;   //if OSD not enabled then stop screen dimming when blank OSD turned on below
+                    flags |= BIT_SKIP_ALT_FRAME;          //force dropping of alternate frames to eliminate tear
+                 //}
 
              }
              wait_for_pi_fieldsync();       //ensure that the source and Pi frames are in a repeatable phase relationship