Add mount system and crash mouse-release handler

Render mount M2 model under player with seated animation, apply creature
skin textures, server-driven speed via SMSG_FORCE_RUN_SPEED_CHANGE, and
/dismount command. X11 XUngrabPointer on crash/hang to always release mouse.

src/ui/game_screen.cpp

@@ -1138,6 +1138,13 @@ void GameScreen::sendChatMessage(game::GameHandler& gameHandler) {
                 return;
             }
 
+            // /dismount command
+            if (cmdLower == "dismount") {
+                gameHandler.dismount();
+                chatInputBuffer[0] = '\0';
+                return;
+            }
+
             // /sit command
             if (cmdLower == "sit") {
                 gameHandler.setStandState(1);  // 1 = sit