Make EGL context init work when msaa is requested but there's no surfaceless

To query back certain things, like the actual OpenGL version from the created context,
the eglconvenience code makes the new context current either without a surface
(in case EGL_KHR_surfaceless_context is supported) or with a pbuffer (or other
platform specific equivalent). There is no problem with the former. The latter
can however fail when samples are requested for the context and there are no
matching pbuffer configs. The result is a pbuffer without multisampling,
which could lead to a BAD_MATCH.

This is visible on GLES 2.0 devices like the Beaglebone. The solution is to fall
back to creating a new, temporary context with a config matching the pbuffer's
whenever the makeCurrent attempt fails.

Change-Id: Icd9f3383f2cad36dd5b60dead655328b7b9af658
Reviewed-by: Andy Nichols <andy.nichols@theqtcompany.com>