Add a test for switching between different contexts across multiple vendors.
The new test also checks libEGL's recovery when a vendor's eglMakeCurrent
fails.
The EGL dummy vendor will now keep track of the current context, so that we
can test whether eglMakeCurrent calls get passed through correctly.
Also added a command to force an eglMakeCurrent call to fail, and to fetch the
the current context from directly from a vendor library.
