shell: correct position of a surface before rotating it.

How to reproduce: rotate the surface to something like 45 degrees,
resize it drastically, continue to rotate. The surface will jump
some space and the rotation point will not be in the center
of the surface.

Fix is to shift the surface position to match the rotation point

src/shell.c

@@ -1284,7 +1284,7 @@ rotate_binding(struct wl_input_device *device, uint32_t time,
 		(struct weston_surface *) device->pointer_focus;
 	struct shell_surface *surface;
 	struct rotate_grab *rotate;
-	GLfloat dx, dy;
+	GLfloat dx, dy, cx, cy, cposx, cposy, dposx, dposy;
 	GLfloat r;
 
 	if (base_surface == NULL)
@@ -1335,6 +1335,20 @@ rotate_binding(struct wl_input_device *device, uint32_t time,
 		weston_matrix_init(&rotate->rotation);
 	}
 
+	/* We need to adjust the position of the surface
+	 * in case it was resized in a rotated state before */
+	cx = 0.5f * surface->surface->geometry.width;
+	cy = 0.5f * surface->surface->geometry.height;
+	cposx = surface->surface->geometry.x + cx;
+	cposy = surface->surface->geometry.y + cy;
+	dposx = rotate->center.x - cposx;
+	dposy = rotate->center.y - cposy;
+	if (dposx != 0 || dposy != 0) {
+		weston_surface_set_position(base_surface,
+				base_surface->geometry.x + dposx,
+				base_surface->geometry.y + dposy);
+	}
+
 	wl_input_device_set_pointer_focus(device, NULL, time, 0, 0);
 }