Clamp keyframe interpolation to last value, add main menu scene/UI, battle HUD, and expanded test coverage

- keyframeGetValue now returns the last keyframe's value for times at or
  beyond it, fixes a missing util/math.h include, and asserts keyframes are
  sorted by time; adds test/animation/test_keyframe.c
- Adds mainmenu scene/UI and a battle HUD UI frame
- Adds save autosave-related fields and battle scene tweaks
- Adds headless test coverage for cutscenes, entities, and map areas

test/rpg/CMakeLists.txt

@@ -10,4 +10,6 @@ dusktest(test_rpg.c)
 
 # Subdirs
 add_subdirectory(overworld)
-add_subdirectory(battle)
+add_subdirectory(battle)
+add_subdirectory(entity)
+add_subdirectory(cutscene)