Fix relative mouse input for Unvanquished (unvanquished.net)

Here's an IRC dump that hopefully explains the issue this fixes:

> I'm debugging something odd where, for a libre game,
  unvanquished.net (a FPS), relative mouse input in fullscreen is
  buggy
> it's like, working mostly ok, but it has a weird
  performance/cleanup bug
> after some time in relative mouse input mode, some time as low
  as 15s, usually more, the SDL sends A LOT of relative mouse
  input per frame
> almost all of which have xrel==0 && yrel==0
> by A LOT, I mean that after ~1min, it's usually in the
  thousands per frame
> each frame, a while ( SDL_PollEvent( &e)) loop reads the
  inputs, but it seems SDL is not clearing the list.
> one way to clear the list is to open the in-game console or
  menu, which switches the input mode to absolute, then close it
  which gets a working relative input mode (for some time at least)
> I've shown the issue to be present with SDL2.0.20 but not with
  2.0.14 on my system
> some other players on Arch Linux (SDL2.0.20) report a possibly
  related issue, where some keys seem to be pressed at random
> I've did some bisection on SDL master, and I've found that
  there are actually two commits involved, one breaking it
  totally (no input at all), and one fixing it partially (with
  the problem described above)

First related commit that breaks it totally:

	commit 82793ac
	Author: Sam Lantinga <[email protected]>
	Date:   Thu Oct 14 14:26:21 2021 -0700

	    Fixed mouse warping while in relative mode

	    We should get a mouse event with an absolute position and no relative motion and shouldn't change the OS cursor position at all

Second related commit, that halfway fixes it:

	commit 31f8c3e
	Author: Sam Lantinga <[email protected]>
	Date:   Thu Jan 6 11:27:44 2022 -0800

	    Fixed event pump starvation if the application frequently pushes its own events

Reverting the first commit did fix the issue for me, but would
probably reintroduce the bug it was fixing(?). This patch should
fix it for everyone hopefully.

DaemonEngine/Daemon#600 is the upstream
bug, and contains some early investigation.

src/events/SDL_mouse.c

@@ -383,6 +383,7 @@ SDL_PrivateSendMouseMotion(SDL_Window * window, SDL_MouseID mouseID, int relativ
         mouse->x = x;
         mouse->y = y;
         mouse->has_position = SDL_TRUE;
+        return 0;
     } else if (!xrel && !yrel) {  /* Drop events that don't change state */
 #ifdef DEBUG_MOUSE
         printf("Mouse event didn't change state - dropped!\n");