[regression] Black server side window decorations with gnome-shell/mutter

Commit dabc7d8b (!319 (merged)) is causing a regression, all window decorations appear black with gnome-shell/mutter.

This is in Mesa DRI Intel(R) UHD Graphics 620 (Kabylake GT2) so that code patch should not be hit, yet it is.

Some further debugging shows that epoxy_gl_version() returns 0 here, which is the error value in epoxy_gl_version().

This is because glGetString(GL_VERSION) return NULL, possibly because the context is not current as this point?

Further calls glGetString(GL_VERSION) later rightfully return OpenGL ES 3.2 Mesa 19.1.8 as expected.