output_init_render: Allow re-initialization

This lets the compositor call this function after the fact to replace
the renderer/allocator after a renderer context lost.

include/wlr/types/wlr_output.h

@@ -278,7 +278,8 @@ void wlr_output_create_global(struct wlr_output *output);
 void wlr_output_destroy_global(struct wlr_output *output);
 /**
  * Initialize the output's rendering subsystem with the provided allocator and
- * renderer. Can only be called once.
+ * renderer. After initialization, this function may invoked again to reinitialize
+ * the allocator and renderer to different values.
  *
  * Call this function prior to any call to wlr_output_attach_render(),
  * wlr_output_commit() or wlr_output_cursor_create().

types/output/render.c

@@ -15,8 +15,8 @@
 
 bool wlr_output_init_render(struct wlr_output *output,
 		struct wlr_allocator *allocator, struct wlr_renderer *renderer) {
-	assert(output->allocator == NULL && allocator != NULL);
-	assert(output->renderer == NULL && renderer != NULL);
+	assert(allocator != NULL && renderer != NULL);
+	assert(output->back_buffer == NULL);
 
 	uint32_t backend_caps = backend_get_buffer_caps(output->backend);
 	uint32_t renderer_caps = renderer_get_render_buffer_caps(renderer);
@@ -31,6 +31,12 @@ bool wlr_output_init_render(struct wlr_output *output,
 		return false;
 	}
 
+	wlr_swapchain_destroy(output->swapchain);
+	output->swapchain = NULL;
+
+	wlr_swapchain_destroy(output->cursor_swapchain);
+	output->cursor_swapchain = NULL;
+
 	output->allocator = allocator;
 	output->renderer = renderer;