scene: add a way to render arbitrary textures

[emersion]

Some compositors like Sway need to render arbitrary textures. This includes use-cases such as:

• Titlebar textures
• Saved surface buffers for atomic layout transactions

The scene-graph API doesn't yet allow to do this. Potential solutions include:

• Add a new TEXTURE node type, which takes a wlr_texture. Probably the most straightforward, but ties the node to a specific renderer and doesn't allow for direct scan-out.
• Add a new BUFFER node type, which takes a wlr_buffer. The scene-graph would be responsible for uploading/importing the buffer as a texture. However, this potentially duplicates the work with wlr_surface's client buffers in the case of displaying a saved surface buffer.
• Add a new CUSTOM node type, which takes an arbitrary rendering function and the extents of the node. This would allow compositors to still use the scene-graph even when they do custom rendering (e.g. 3D effects). This doesn't allow direct scan-out and could go overboard.