render: introduce wlr_render_texture_pass

Split texture rendering out of wlr_render_pass and introduce a
dedicated wlr_render_texture_pass interface.

Remove the add_texture hook from wlr_render_pass_impl and route
texture rendering through renderer->texture_pass instead.

types/output/render.c

@@ -37,6 +37,7 @@ bool wlr_output_init_render(struct wlr_output *output,
 	output->renderer = renderer;
 
 	get_or_create_render_rect_pass(renderer);
+	get_or_create_render_texture_pass(renderer);
 
 	wl_signal_emit_mutable(&output->events.render_inited, output);