Merge #4232

4232: Still clear pending mouse motion r=def- a=Jupeyy

Same as #3932 ... again :/

## Checklist

- [x] Tested the change ingame
- [ ] Provided screenshots if it is a visual change
- [ ] Tested in combination with possibly related configuration options
- [ ] Written a unit test if it works standalone, system.c especially
- [ ] Considered possible null pointers and out of bounds array indexing
- [ ] Changed no physics that affect existing maps
- [ ] Tested the change with [ASan+UBSan or valgrind's memcheck](https://github.com/ddnet/ddnet/#using-addresssanitizer--undefinedbehavioursanitizer-or-valgrinds-memcheck) (optional)


Co-authored-by: Jupeyy <jupjopjap@gmail.com>

src/engine/client/input.cpp

@@ -469,6 +469,11 @@ int CInput::Update()
 					if(GetMouseMode() != INPUT_MOUSE_MODE_ABSOLUTE)
 					{
 						SDL_ShowCursor(SDL_FALSE);
+						if(SDL_GetRelativeMouseMode())
+						{
+							// Clear pending relative mouse motion
+							SDL_GetRelativeMouseState(0x0, 0x0);
+						}
 					}
 					m_MouseFocus = true;
 					IgnoreKeys = true;