Do not abort EGL backend creation if EGL_NV_post_sub_buffer isn't supported

In case the extension is not present eglQuerySurface returns EGL_FALSE
when querying for EGL_POST_SUB_BUFFER_SUPPORTED_NV and sets an
EGL_BAD_ATTRIBUTE error state. If this is the case it's not an error we
should abort on, but one we should ignore as it's the same as extension
not supported.


FIXED: 4.10.1