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