Make VBlank flags hew more closely to console behavior

src/ppu.c

@@ -25,6 +25,7 @@ uint8_t nes_ppu_read(nes_ppu* ppu, uint16_t addr) {
     if (ppu_reg_status == addr) {
         val = ppu->status;
         ppu->latch = 0;
+        ppu->status &= ~ppu_Status_VBlank;
 
     } else if (oam_reg_data == addr) {
         OAM_LOG("PPU: OAM READ %02x > %02x\n", ppu->oam_addr, val);
@@ -274,13 +275,13 @@ nes_ppu_Result nes_ppu_run(nes_ppu* ppu, int cycles) {
             ppu->frame++;
             result = ppu_Result_VBlank_Off;
 
-        } else if (ppu->scanline >= nes_ppu_active) {
+        } else if (ppu->scanline == nes_ppu_active) {
             ppu->status |= ppu_Status_VBlank;
             if (ppu->control & ppu_Control_VBlank) {
                 result = ppu_Result_VBlank_On;
             }
 
-        } else {
+        } else if (ppu->scanline < nes_ppu_active) {
             if (    ppu->scanline > nes_ppu_prerender &&
                     ppu->scanline < nes_ppu_render) {
                 ppu->scroll_y++;