wayland/egl: Ensure correct buffer size when allocating

Whenever a buffer is allocated, e.g. by the first draw call or EGL call after a
buffer swap, make sure the size is up to date. Prior to this commit, we
failed to do so when querying the buffer age, or swapping buffers
without any prior EGL call or draw call.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 903ad594)