compositor: Set EGL_PLATFORM env variable for each backend.

compositor: Set EGL_PLATFORM env variable for each backend.

I may have missed something, but - since the Wayland compositor
already picks a platform backend, opens a connection and initializes the
backend specific display data structure it doesn't make sense
to let egl pick a platform. If it picks a different one the
display specific data structure will most likely not match.
Thus determine the platform in the Wayland rendering backend by setting
the EGL_PLATFORM env variable.
For the client any other platform than 'wayland' doesn't seem to make
sense.
I'm not sure if I've got the the platform ofr openfwd right.

Signed-off-by: Egbert Eich <eich@freedesktop.org>