egl: Add a 565 pbuffer-only EGL config under X11.

egl: Add a 565 pbuffer-only EGL config under X11.

The CTS requires a 565-no-depth-no-stencil (meaning d/s not-required, not
not-present) config for ES 3.0, but at depth 24 of X11 we wouldn't do so.
We can satisfy that bad requirement using a pbuffer-only visual with
whatever other buffers the driver happens to have given us.

I've tried to raise this as an absurd requirement with Khronos and made no
progress.

v2: Make sure it's single sample, no depth, no stencil.  Comment typo fix

Reviewed-by: Adam Jackson <ajax@redhat.com>