egl/glvnd: add a check to make sure GLVND and plain EGL always expose the same entrypoints
@kbrenneman was there a reason for the EGL_KHR_debug
entrypoints not to be in the GLVND dispatch list, or were they simply forgotten?
@evelikov mentioned that maybe they should only exist in GLVND itself and not in the dispatch table, but I don't see how that would work... but then again, I don't really understand much of GLVND
As explained below, this extension's entrypoints should not be listed in the GLVND entrypoints, so instead that's what the check now does with them.
/cc @tpalli who suggested a test be made, and @daniels who started the discussion