Merge #3932

3932: Reset relative mouse motion on toggle r=def- a=Jupeyy

<!-- What is the motivation for the changes of this pull request -->

## 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

@@ -89,6 +89,8 @@ void CInput::MouseModeRelative()
 	m_InputGrabbed = 1;
 	SDL_SetRelativeMouseMode(SDL_TRUE);
 	Graphics()->SetWindowGrab(true);
+	// Clear pending relative mouse motion
+	SDL_GetRelativeMouseState(0x0, 0x0);
 }
 
 int CInput::MouseDoubleClick()
@@ -334,8 +336,6 @@ int CInput::Update()
 						MouseModeRelative();
 					m_MouseFocus = true;
 					IgnoreKeys = true;
-					// We should do this call to reset relative mouse position after alt+tab
-					SDL_GetRelativeMouseState(0x0, 0x0);
 					break;
 				case SDL_WINDOWEVENT_FOCUS_LOST:
 					m_MouseFocus = false;