Group mouse move events from C API function

Groups together consecutive mouse move events like done in core.step()
lua function but on the C side.

It does not introduce any meaningful speedup but it theory is more efficient and
simplifies the Lua code.

The simplification of the Lua code alone is enough to justify this change?

src/api/system.c

@@ -201,6 +201,14 @@ top:
       return 4;
 
     case SDL_MOUSEMOTION:
+      SDL_PumpEvents();
+      SDL_Event event_plus;
+      while (SDL_PeepEvents(&event_plus, 1, SDL_GETEVENT, SDL_MOUSEMOTION, SDL_MOUSEMOTION) > 0) {
+        e.motion.x = event_plus.motion.x;
+        e.motion.y = event_plus.motion.y;
+        e.motion.xrel += event_plus.motion.xrel;
+        e.motion.yrel += event_plus.motion.yrel;
+      }
       lua_pushstring(L, "mousemoved");
       lua_pushnumber(L, e.motion.x);
       lua_pushnumber(L, e.motion.y);