Fix backdrop override quirk

src/ppu.c

@@ -89,6 +89,10 @@ static inline void nes_ppu_internal_copy_y(nes_ppu* ppu) {
                     ((ppu->t & 0x7000U) >> 12);
 }
 
+static inline void nes_ppu_internal_copy_v(nes_ppu* ppu) {
+    ppu->addr = ppu->t;
+}
+
 void nes_ppu_write(nes_ppu* ppu, uint16_t addr, uint8_t val) {
     PPU_LOG("PPU: W-> $%04x %02x\n", addr, val);
 
@@ -236,6 +240,9 @@ nes_ppu_Result nes_ppu_run(nes_ppu* ppu, int cycles) {
         if (ppu->scanline == 0) {
             nes_ppu_internal_copy_y(ppu);
         }
+        if (ppu->mask & (ppu_Mask_Sprite | ppu_Mask_Back)) {
+            nes_ppu_internal_copy_v(ppu);
+        }
     }
 
     if (    NULL != ppu->mapper->scanline &&

src/sdl_render.c

@@ -560,7 +560,7 @@ static void render_scanline(nes_ppu* ppu, int line,
 
     if (line >= 0) {
         // Emulate the happy part of the backdrop override quirk
-        int pal_idx = (ppu->addr >= nes_ppu_mem_pal_start) ?
+        int pal_idx = ((ppu->addr & 0x3FFFU) >= nes_ppu_mem_pal_start) ?
                       (ppu->addr & (nes_ppu_mem_pal_size - 1)) : 0;
         SDL_Color ext = nes_palette[ppu->palette[pal_idx]];
         SDL_FillRect(data->target, &dst_rect, ((int)ext.r << 16) |