Port fix for getting mouse stuck in box

Apparently, the problem is caused by this issue:
ValveSoftware/halflife#1377

Which Solokiller has a fix for:
twhl-community/halflife-updated@92ffa23

But seems to be originated from L453rh4wk:
ValveSoftware/halflife#1546 (comment)

cl_dll/inputw32.cpp

@@ -156,6 +156,7 @@ DWORD	s_hMouseThreadId = 0;
 HANDLE	s_hMouseThread = 0;
 HANDLE	s_hMouseQuitEvent = 0;
 HANDLE	s_hMouseDoneQuitEvent = 0;
+SDL_bool mouseRelative = SDL_TRUE;
 #endif
 
 /*
@@ -235,6 +236,21 @@ void DLLEXPORT IN_ActivateMouse (void)
 #endif
 		mouseactive = 1;
 	}
+
+#ifdef _WIN32
+	if (!m_bRawInput)
+	{
+		SDL_SetRelativeMouseMode(SDL_FALSE);
+		mouseRelative = SDL_FALSE;
+	}
+	else
+	{
+		mouseRelative = SDL_TRUE;
+		SDL_SetRelativeMouseMode(SDL_TRUE);
+	}
+#else
+	SDL_SetRelativeMouseMode(SDL_TRUE);
+#endif
 }
 
 
@@ -255,6 +271,15 @@ void DLLEXPORT IN_DeactivateMouse (void)
 
 		mouseactive = 0;
 	}
+
+#ifdef _WIN32
+	if (m_bRawInput)
+	{
+		mouseRelative = SDL_FALSE;
+	}
+
+#endif
+	SDL_SetRelativeMouseMode(SDL_FALSE);
 }
 
 /*
@@ -572,6 +597,19 @@ void IN_MouseMove ( float frametime, usercmd_t *cmd)
 
 	gEngfuncs.SetViewAngles( (float *)viewangles );
 
+#ifdef _WIN32
+	if (!m_bRawInput && mouseRelative)
+	{
+		SDL_SetRelativeMouseMode(SDL_FALSE);
+		mouseRelative = SDL_FALSE;
+	}
+	else if (m_bRawInput && !mouseRelative)
+	{
+		SDL_SetRelativeMouseMode(SDL_TRUE);
+		mouseRelative = SDL_TRUE;
+	}
+#endif
+
 /*
 //#define TRACE_TEST
 #if defined( TRACE_TEST )