Replace/remove animation_base, animation, animator, and screen_transi…

…tion classes.

src/engine/animation/animation-player.cpp

@@ -2,9 +2,13 @@
 // SPDX-License-Identifier: GPL-3.0-or-later
 
 #include <engine/animation/animation-player.hpp>
+#include <algorithm>
 
 void animation_player::advance(float seconds)
 {
+	// Prevent negative timesteps
+	seconds = std::max(0.0f, seconds);
+
 	if (!m_sequence)
 	{
 		// No active animation sequence, advance position and return

src/engine/animation/animation-sequence.cpp

@@ -14,7 +14,7 @@ void animation_sequence::trigger_cues(float start_time, float end_time, animatio
 {
 	const auto start_it = m_cues.lower_bound(start_time);
 	const auto end_it = m_cues.upper_bound(end_time);
-	
+
 	for (auto it = start_it; it != end_it; ++it)
 	{
 		// Make end time exclusive