xwayland: make context current to check GL version

`glGetString(GL_VERSION)` will return NULL without a current context.

Commit dabc7d8b (“xwayland: Fall back to GLES2 if we don't get at least
GL 2.1 in glamor”) would check the context is created, but it is made
current just after, so the call to `epoxy_gl_version()` would return 0,
hence defeating the version check.

Make the context current prior to call `epoxy_gl_version()`.

Fixes: dabc7d8b - xwayland: Fall back to GLES2 if we don't get at least
                  GL 2.1 in glamor
Closes: #932
!324
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
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