xdg-shell: Add set_margin request

This is used to figure out the size of "invisible" decorations, which we'll
use to better know the visible extents of the surface, which we can use for
constraining, titlebars, and more.

shared/cairo-util.h

@@ -165,6 +165,9 @@ void
 frame_opaque_rect(struct frame *frame, int32_t *x, int32_t *y,
 		  int32_t *width, int32_t *height);
 
+int
+frame_get_shadow_margin(struct frame *frame);
+
 uint32_t
 frame_status(struct frame *frame);
 

shared/frame.c

@@ -578,6 +578,14 @@ frame_opaque_rect(struct frame *frame, int32_t *x, int32_t *y,
 		*height = frame->height - frame->opaque_margin * 2;
 }
 
+int
+frame_get_shadow_margin(struct frame *frame)
+{
+	frame_refresh_geometry(frame);
+
+	return frame->shadow_margin;
+}
+
 uint32_t
 frame_status(struct frame *frame)
 {