clients/nested: use weston_check_egl_extension over strstr
clients/nested: use weston_check_egl_extension over strstr

Signed-off-by: Emil Velikov <f20878473f414da183da19ee25c8722cc1d711e4@collabora.com>
Reviewed-by: Daniel Stone <62e7e355d5d8400fa8384448796af0bf6be4ffae@collabora.com>

krezovic/weston,sir-murray/weston,sir-murray/weston,krezovic/weston