Implement step-wise animation locked to time series

animation.h

@@ -5,14 +5,16 @@
 
 typedef struct {
 	mat4_t *bone_matrices;
-	bone_t *current;
-	unsigned keyframe;
+	const bone_t *current, *next;
+	uint32_t current_keyframe, next_keyframe;
+	uint32_t duration_us, elapsed_us;
 } animation_state_t;
 
 animation_state_t animation_init(const model_t *model);
 void animation_cleanup(animation_state_t *state);
 
 void animation_start(animation_state_t *state, const model_t *model);
-void animation_update(animation_state_t *state, const model_t *model);
+void animation_update(
+    animation_state_t *state, const model_t *model, uint32_t dt_us);
 
 #endif