Force composition for some layers

[emersion]

Some compositors might want to draw e.g. window decorations with OpenGL, or other textures. How should this be achieved?

[emersion]

Another use-case is when the compositor doesn't have a KMS framebuffer ID for a client - either because:

• The client is using software rendering
• The format/modifier is supported for rendering but not for direct scan-out
• The GPU buffer is not scan-out capable