compositor: Adapt to wl_surface going away

struct weston_surface is now the only surface type we have (in core, shell.c
has shell_surface, of course).  A lot of code gets simpler and we never
have to try to guess whether an API takes a wl_surface or a weston_surface.
diff --git a/src/text-backend.c b/src/text-backend.c
index 036d48a..92efd9f 100644
--- a/src/text-backend.c
+++ b/src/text-backend.c
@@ -39,7 +39,7 @@
 
 	struct wl_list input_methods;
 
-	struct wl_surface *surface;
+	struct weston_surface *surface;
 
 	pixman_box32_t cursor_rectangle;
 
@@ -794,7 +794,7 @@
 	struct weston_keyboard *keyboard = data;
 	struct input_method *input_method =
 		container_of(listener, struct input_method, keyboard_focus_listener);
-	struct wl_surface *surface = keyboard->focus;
+	struct weston_surface *surface = keyboard->focus;
 
 	if (!input_method->model)
 		return;