Add support for left background column mask

src/sdl_render.c

@@ -326,8 +326,6 @@ static inline void render_bg_scanline_area(
 
 static void render_bg_scanline(const nes_ppu* ppu, int scanline,
                                uint8_t* dst) {
-    // TODO: Handle column 0 flag
-
     int page = (ppu->control & ppu_Control_Nametable_Mask);
     int x = ppu->scroll_x;
     int y = ppu->scroll_y + scanline;
@@ -338,8 +336,16 @@ static void render_bg_scanline(const nes_ppu* ppu, int scanline,
     }
 
     int w = (nes_ppu_render_w - x);
+    if (!(ppu->mask & ppu_Mask_Left_Back)) {
+        // Handle column 0 flag - need to fill with transparency
+        memset(dst, 0xFFU, 8);
+        w -= 8;
+        x += 8;
+        dst += 8;
+    }
     render_bg_scanline_area(ppu, page, dst, x, y, w);
-    render_bg_scanline_area(ppu, page ^ 1, dst + w, 0, y, x);
+    render_bg_scanline_area(ppu, page ^ 1, dst + w, 0, y,
+                            nes_ppu_render_w - w);
 }
 
 /*