Compositors now have more control over how the backend creates its
renderer. Currently all backends create an EGL/GLES2 renderer, so
the necessary attributes for creating the context are passed to a
user-provided callback function. It is responsible for initializing
provided wlr_egl and to return a renderer. On fail, return 0.
Fixes#987
compositor_fini destroys the display, but it is an error to destroy it
before e.g. wlr_seat that references it.
This lets us order destroy calls properly, following first-in-last-out
logic.
Ilia Bozhinov
Timidger
emersion
Drew DeVault
Dominique Martinet
Scott Anderson