shell: Move weston_curtain_create params into the struct

Given that we have a struct for argument params, we might as well use it
rather than have them split between the struct and native params. For
consistency between the implementations, this also includes a shift from
float to int positioning for the base offset within the compositor's
global co-ordinate space.

Signed-off-by: Daniel Stone <daniels@collabora.com>

tests/safe-signal-output-removal-test.c

@@ -81,14 +81,15 @@ output_create_view(struct test_output *t_output)
 {
        struct weston_curtain_params curtain_params = {
 	       .r = 0.5, .g = 0.5, .b = 0.5,
+	       .x = 0, .y = 0,
+	       .width = 320, .height = 240,
 	       .get_label = NULL,
 	       .surface_committed = NULL,
 	       .surface_private = NULL,
        };
 
        t_output->view = weston_curtain_create(t_output->compositor,
-					      &curtain_params,
-					      0, 0, 320, 240);
+					      &curtain_params);
        weston_view_set_output(t_output->view, t_output->output);
 
        /* weston_compositor_remove_output() has to be patched with