Fix compilation errors with old SDL versions

SDL_PixelFormatEnum has been introduced in SDL 2.0.10:
<libsdl-org/SDL@cc6a8ac>

SDL_PIXELFORMAT_BGR444 has been introduced in SDL 2.0.12:
<libsdl-org/SDL@a1c1185>

Fixes #2777 <#2777>
PR #2781 <#2781>

Reviewed-by: Yu-Chen Lin <npes87184@gmail.com>

app/src/icon.c

@@ -158,6 +158,12 @@ decode_image(const char *path) {
     return result;
 }
 
+#if !SDL_VERSION_ATLEAST(2, 0, 10)
+// SDL_PixelFormatEnum has been introduced in SDL 2.0.10. Use int for older SDL
+// versions.
+typedef int SDL_PixelFormatEnum;
+#endif
+
 static SDL_PixelFormatEnum
 to_sdl_pixel_format(enum AVPixelFormat fmt) {
     switch (fmt) {
@@ -172,7 +178,9 @@ to_sdl_pixel_format(enum AVPixelFormat fmt) {
         case AV_PIX_FMT_BGR565BE: return SDL_PIXELFORMAT_BGR565;
         case AV_PIX_FMT_BGR555BE: return SDL_PIXELFORMAT_BGR555;
         case AV_PIX_FMT_RGB444BE: return SDL_PIXELFORMAT_RGB444;
+#if SDL_VERSION_ATLEAST(2, 0, 12)
         case AV_PIX_FMT_BGR444BE: return SDL_PIXELFORMAT_BGR444;
+#endif
         case AV_PIX_FMT_PAL8: return SDL_PIXELFORMAT_INDEX8;
         default: return SDL_PIXELFORMAT_UNKNOWN;
     }