wsegl: simplify pixmap handling
wsegl: support creation of buffers using wl_shm
wsegl: force creation of buffers using shm with an env variable
wsegl: support swap interval value of 0

For simplicity the max swap interval is now 1. The maximum number of
buffers has been upped to 4, to handle the case where the client buffer
is used as scanout.
gdl: post error when resource cannot be created
gdl: replace deprecated wayland api