Since wlroots shaders only use one texture at a time (ie there is only one
sampler2D variable in any shader), it is unnecessary to switch between active
texture units at this time.
Add a signal for wlr_surface destruction on the wlr_surface that compositors
can listen to to remove the surface from their state.
Implement a listener for this in the example wl_compositor to remove the
surface from its internal list of surfaces.
Destroy the surface in the compositor destroy_surface callback given when the
surface resource was created.
Add a reference to the surface resource to the wlr_surface so a compositor can
find it in its list of resources upon wlr_resource destruction.
Implement surface_attach method. This is called when a client attaches an shm
buffer with wl_surface_attach().
Implement the GLES2 interface for attaching shm buffers. This creates an opengl
texture with the shm buffer contents for the surface.
This commit also includes some working code to render the surfaces onto the
screen for demonstration purposes.
