render: guard rendering operations between begin() and end()

Add a wlr_renderer.rendering bool, set it to true between
wlr_renderer_begin() and wlr_renderer_end(). Assert we're rendering when
calling functions that render.

include/wlr/render/wlr_renderer.h

@@ -25,6 +25,8 @@ struct wlr_drm_format_set;
 struct wlr_renderer {
 	const struct wlr_renderer_impl *impl;
 
+	bool rendering;
+
 	struct {
 		struct wl_signal destroy;
 	} events;