Implement EGL_ANGLE_context_virtualization on GLX

Implement the EGL_ANGLE_context_virtualization extension that,
despite its name, actually allows the de-virtualization of EGL
contexts.

Implement ContextGLX and RendererGLX classes to hold the
native glx context per-renderer, rather than per-DisplayGLX
as before. Where ContextGL and RendererGL were created, create
these new subclasses instead.

Remove the glx::Context and glx::Pbuffer from DisplayGLX in
favour of the ones in RendererGLX instead.

Factor a new DisplayGLX::createRenderer() out of
DisplayGLX::initialize(). This creates a glx::Context and
glx::Pbuffer for the new Renderer. Implement
DisplayGLX::mVirtualizationGroups to store a map from virtualization
group to Renderer.

Add an EGLContext test that exercises eglMakeCurrent and
eglGetCurrentContext on multiple threads.

Add a parameter to EGLWindow to request EGL_PBUFFER_BIT.
Some tests were silently relying on the backend choosing a config
that supports pbuffers, but GLX does not. Modify the tests to
explicitly request support

Bug: angleproject:40096700, chromium:491346041
Change-Id: I71b309c44f48e956e67794daf2ba067bd9fbfbad
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/7688012
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>