[GEN] avoid a flickering frame if a game enables interlacing with a n…

…on-blank frame

backend/genesis-core/src/vdp.rs

@@ -915,11 +915,16 @@ impl Vdp {
                 self.state.frame_count += 1;
                 self.state.v_border_forgotten = false;
 
-                self.state.interlaced_frame = match self.registers.interlacing_mode {
+                let next_frame_interlaced = match self.registers.interlacing_mode {
                     InterlacingMode::Progressive => false,
                     InterlacingMode::Interlaced => !self.config.deinterlace,
                     InterlacingMode::InterlacedDouble => true,
                 };
+                if next_frame_interlaced && !self.state.interlaced_frame && !self.config.deinterlace
+                {
+                    self.prepare_frame_buffer_for_interlaced();
+                }
+                self.state.interlaced_frame = next_frame_interlaced;
 
                 // Top border length needs to be saved at start-of-frame in case there is a mid-frame swap between V28
                 // mode and V30 mode. Titan Overdrive 2 depends on this for the arcade scene
@@ -1055,6 +1060,20 @@ impl Vdp {
         }
     }
 
+    fn prepare_frame_buffer_for_interlaced(&mut self) {
+        // Duplicate every line to avoid a flickering frame if a game enables interlacing without
+        // first blanking the screen
+        let screen_width = self.screen_width();
+        let screen_height = self.screen_height();
+        for scanline in (0..screen_height).rev() {
+            for pixel in 0..screen_width {
+                let color = self.frame_buffer[(scanline * screen_width + pixel) as usize];
+                self.frame_buffer[((2 * scanline) * screen_width + pixel) as usize] = color;
+                self.frame_buffer[((2 * scanline + 1) * screen_width + pixel) as usize] = color;
+            }
+        }
+    }
+
     fn in_vblank(&self) -> bool {
         self.state.scanline >= self.registers.vertical_display_size.active_scanlines()
             && self.state.scanline < self.timing_mode.scanlines_per_frame() - 1