GPU2D: fill gaps in BG modes

* mode6 actually works on the sub GPU, albeit limited to 1/4 the full bitmap size due to having only 128K of VRAM
* mode7 draws BG0, BG1 and sprites. no BG2/BG3.

src/GPU2D.h

@@ -135,6 +135,7 @@ private:
 
     template<u32 bgmode> void DrawScanlineBGMode(u32 line);
     void DrawScanlineBGMode6(u32 line);
+    void DrawScanlineBGMode7(u32 line);
     void DrawScanline_BGOBJ(u32 line);
 
     static void DrawPixel_Normal(u32* dst, u16 color, u32 flag);