Implement 'negative Y' scrolling

- TMNT now working as expected

Makefile

@@ -30,7 +30,7 @@ PFLAGS += -g
 #PFLAGS += -DDEBUG_APU
 #PFLAGS += -DDEBUG_INPUT
 #PFLAGS += -DE6502_DEBUG
-PFLAGS += -DE6502_ILLEGAL
+#PFLAGS += -DE6502_ILLEGAL
 CFLAGS += $(PFLAGS) -Wall -Werror -Wshadow -Wunused -I../ -Isrc/
 CFLAGS += -Iinc/$(PLATFORM)/SDL2
 LDFLAGS += $(PFLAGS)

src/ppu.c

@@ -277,8 +277,8 @@ nes_ppu_Result nes_ppu_run(nes_ppu* ppu, int cycles) {
             if (    ppu->scanline > nes_ppu_prerender &&
                     ppu->scanline < nes_ppu_render) {
                 ppu->scroll_y++;
-                if (ppu->scroll_y >= nes_ppu_render_h) {
-                    ppu->scroll_y -= nes_ppu_render_h;
+                if (ppu->scroll_y == nes_ppu_render_h) {
+                    ppu->scroll_y = 0;
                     ppu->control ^= 0b10;
                 }
             }

src/sdl_render.c

@@ -362,6 +362,7 @@ static void render_bg_scanline(const nes_ppu* ppu,/* int scanline,*/
     int page = (ppu->control & ppu_Control_Nametable_Mask);
     int x = ppu->scroll_x;
     int y = ppu->scroll_y;
+    if (y >= nes_ppu_render_h) y -= nes_ppu_render_h;
     int w = (nes_ppu_render_w - x);
     if (!(ppu->mask & ppu_Mask_Left_Back)) {
         // Handle column 0 flag - need to fill with transparency