Take the tutorial figures out of the games themselves

Both examples gain --screenshot PATH and --screenshot-frame N. The capture sits
between the world being drawn and the frame being presented, because
SDL_RenderPresent is where the target stops being readable, and it works under
the dummy video driver and the software renderer like the rest of the headless
path does.

`cmake --build build --target docs_game_figures` regenerates
docs/images/sidescroller.png and docs/images/jrpg.png by running each game to a
chosen frame. Same contract as docs_screenshots: deliberate, never part of a
build, because the PNGs are tracked. There is no --check counterpart, and the
target says why -- the sidescroller drives its physics from the wall clock, so a
byte comparison would fail for reasons that have nothing to do with the
documentation.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KzBDV2fqgnUAcqCKqKvc71

docs/21-tutorial-jrpg.md

@@ -547,7 +547,6 @@ zero gravity, which is what the property defaults already give you.
     PASS(errctx, akgl_renderer->frame_start(akgl_renderer));
     PASS(errctx, akgl_game_update(&opflags));
     PASS(errctx, jrpg_textbox_draw());
-    PASS(errctx, akgl_renderer->frame_end(akgl_renderer));
 ```
 
 `akgl_game_update` is update-every-actor, step the physics, draw the world. It