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.cpp

@@ -274,6 +274,8 @@ void Game::init(void)
 
     deathcounts = 0;
     gameoverdelay = 0;
+    framecounter = 0;
+    seed_use_sdl_getticks = false;
     resetgameclock();
     gamesaved = false;
     gamesavefailed = false;