It worked (for the specific case in the bug report), but for a more
generic fix that covers also other similar cases (where VCL code gets
called in threads that epoxy doesn't know about, or something), it's
better to call epoxy_handle_external_wglMakeCurrent() in vcl, instead
of all over the place in thread callbacks.

This reverts commit 78c67c6bbba0eaa418c573637858828b274b3c72.

Change-Id: Iaad7b3d1191847b9d3f1e74c3339de9f16194aed