add GetSnesPixelFormat and SNES_PIXELFORMAT_XBPP constants

src/app/gfx/bitmap.cc

@@ -46,6 +46,20 @@ void PngReadCallback(png_structp png_ptr, png_bytep outBytes,
     png_error(png_ptr, "Read error in PngReadCallback");
   }
 }
+
+Uint32 GetSnesPixelFormat(int format) {
+  switch (format) {
+    case 0:
+      return SDL_PIXELFORMAT_INDEX8;
+    case 1:
+      return SNES_PIXELFORMAT_2BPP;
+    case 2:
+      return SNES_PIXELFORMAT_4BPP;
+    case 3:
+      return SNES_PIXELFORMAT_8BPP;
+  }
+  return SDL_PIXELFORMAT_INDEX8;
+}
 }  // namespace
 
 bool ConvertSurfaceToPNG(SDL_Surface *surface, std::vector<uint8_t> &buffer) {

src/app/gfx/bitmap.h

@@ -22,6 +22,24 @@ namespace app {
  */
 namespace gfx {
 
+constexpr int SNES_PIXELFORMAT_2BPP = SDL_DEFINE_PIXELFORMAT(
+    /*type=*/SDL_PIXELTYPE_INDEX8, /*order=*/0,
+    /*layouts=*/0, /*bits=*/2, /*bytes=*/1);
+
+constexpr int SNES_PIXELFORMAT_4BPP = SDL_DEFINE_PIXELFORMAT(
+    /*type=*/SDL_PIXELTYPE_INDEX8, /*order=*/0,
+    /*layouts=*/0, /*bits=*/4, /*bytes=*/1);
+
+constexpr int SNES_PIXELFORMAT_8BPP = SDL_DEFINE_PIXELFORMAT(
+    /*type=*/SDL_PIXELTYPE_INDEX8, /*order=*/0,
+    /*layouts=*/0, /*bits=*/8, /*bytes=*/1);
+
+// SDL_PIXELFORMAT_INDEX8 =
+// SDL_DEFINE_PIXELFORMAT(SDL_PIXELTYPE_INDEX8, 0, 0, 8, 1),
+
+constexpr int kFormat4bppIndexed = 1;
+constexpr int kFormat8bppIndexed = 2;
+
 /**
  * @brief Convert SDL_Surface to PNG image data.
  */