viewporter: add doc comment explaining compositor requirements
#include <wayland-server-core.h>
+/**
+ * Implementation for the viewporter protocol.
+ *
+ * When enabling viewporter, compositors need to update their rendering logic:
+ *
+ * - The size of the surface texture may not match the surface size anymore.
+ * Compositors must use the surface size only.
+ * - Compositors must call wlr_render_subtexture_with_matrix when rendering a
+ * surface texture with the source box returned by
+ * wlr_surface_get_buffer_source_box.
+ */
struct wlr_viewporter {
struct wl_global *global;
