scene: try to import buffers as textures before rendering

The wlroots APIs currently don't allow importing/uploading a buffer during rendering operations. Scene-graph buffer nodes need to turn their wlr_buffer into a wlr_texture at some point. It's not always possible to do so at wlr_scene_buffer creation time because the scene-graph may have zero outputs at this point, thus no way to grab a wlr_renderer.

Instead, add scene-graph buffers to a pending list and try to import them in wlr_scene_output_commit.

References: #3354