Seed with frame counter instead of SDL_GetTicks

Using SDL_GetTicks() to seed the Gravitron RNG caused many
reproducibility issues while syncing https://tasvideos.org/7575S . To
fix this, add a frame counter, which is a number that is incremented
every frame and never resets, and use it instead.

If someone needs to switch back to SDL_GetTicks() for old TASes, then
provide the -seed-use-sdl-getticks command-line option for them.

desktop_version/src/Game.h

@@ -257,6 +257,8 @@ public:
     int lastsaved;
     int deathcounts;
 
+    int framecounter;
+    bool seed_use_sdl_getticks;
     int frames, seconds, minutes, hours;
     bool gamesaved;
     bool gamesavefailed;