Implement SDL_RWsize for SDL1.2

Source/utils/sdl2_to_1_2_backports.cpp

@@ -512,6 +512,22 @@ char *readSymLink(const char *path)
 #endif
 } // namespace
 
+Sint64 SDL_RWsize(SDL_RWops *context)
+{
+	Sint64 pos = SDL_RWseek(context, 0, RW_SEEK_CUR);
+	if (pos == -1)
+		return -1;
+
+	Sint64 size = SDL_RWseek(context, 0, RW_SEEK_END);
+	if (size == -1)
+		return -1;
+
+	if (SDL_RWseek(context, pos, RW_SEEK_SET) == -1)
+		return -1;
+
+	return size;
+}
+
 char *SDL_GetBasePath()
 {
 	// From sdl2-2.0.9/src/filesystem/unix/SDL_sysfilesystem.c

Source/utils/sdl2_to_1_2_backports.h

@@ -309,5 +309,7 @@ int SDL_BlitScaled(SDL_Surface *src, SDL_Rect *srcrect,
 
 //== Filesystem
 
+Sint64 SDL_RWsize(SDL_RWops *context);
+
 char *SDL_GetBasePath();
 char *SDL_GetPrefPath(const char *org, const char *app);