Add configurable mouse sensitivity

The `--mouse-sensitivity`/`-s` command line option allows the user to
set the value `g_mouseSensitivity` which is used to scale the mouse
movement in `wlserver_mousemotion`.

src/main.cpp

@@ -55,6 +55,7 @@ const struct option *gamescope_options = (struct option[]){
 	{ "rt", no_argument, nullptr, 0 },
 	{ "prefer-vk-device", required_argument, 0 },
 	{ "expose-wayland", no_argument, 0 },
+	{ "mouse-sensitivity", required_argument, nullptr, 's' },
 
 	{ "headless", no_argument, 0 },
 
@@ -146,6 +147,7 @@ const char usage[] =
 	"                                     nis => NVIDIA Image Scaling v1.0.3\n"
 	"  --sharpness, --fsr-sharpness   upscaler sharpness from 0 (max) to 20 (min)\n"
 	"  --expose-wayland               support wayland clients using xdg-shell\n"
+	"  -s, --mouse-sensitivity        multiply mouse movement by given decimal number\n"
 	"  --headless                     use headless backend (no window, no DRM output)\n"
 	"  --cursor                       path to default cursor image\n"
 	"  -R, --ready-fd                 notify FD when ready\n"
@@ -247,6 +249,8 @@ bool g_bHeadless = false;
 
 bool g_bGrabbed = false;
 
+float g_mouseSensitivity = 1.0;
+
 GamescopeUpscaleFilter g_upscaleFilter = GamescopeUpscaleFilter::LINEAR;
 GamescopeUpscaleScaler g_upscaleScaler = GamescopeUpscaleScaler::AUTO;
 
@@ -521,6 +525,9 @@ int main(int argc, char **argv)
 			case 'g':
 				g_bGrabbed = true;
 				break;
+			case 's':
+				g_mouseSensitivity = atof( optarg );
+				break;
 			case 0: // long options without a short option
 				opt_name = gamescope_options[opt_index].name;
 				if (strcmp(opt_name, "help") == 0) {

src/main.hpp

@@ -23,6 +23,8 @@ extern bool g_bFullscreen;
 
 extern bool g_bGrabbed;
 
+extern float g_mouseSensitivity;
+
 enum class GamescopeUpscaleFilter : uint32_t
 {
     LINEAR = 0,

src/wlserver.cpp

@@ -1440,8 +1440,23 @@ void wlserver_mousemotion( int x, int y, uint32_t time )
 	auto server = steamcompmgr_get_focused_server();
 	if ( server != NULL )
 	{
-		XTestFakeRelativeMotionEvent( server->get_xdisplay(), x, y, CurrentTime );
+		float dx = (float) x * g_mouseSensitivity;
+		float dy = (float) y * g_mouseSensitivity;
+
+		// Accumulate sub-pixel increments to avoid jerky cursor movement
+		static float accum_x = 0.0;
+		static float accum_y = 0.0;
+
+		accum_x += modff( dx, &dx );
+		accum_y += modff( dy, &dy );
+
+		XTestFakeRelativeMotionEvent( server->get_xdisplay(), (int) (dx + accum_x), (int) (dy + accum_y), CurrentTime );
 		XFlush( server->get_xdisplay() );
+
+		// Integer value from `modff` is ignored, but argument can't be NULL
+		float _;
+		accum_x = modff( accum_x, &_ );
+		accum_y = modff( accum_y, &_ );
 	}
 }