intel: Call intel_prepare_render() in intelMakeCurrent()

This restores old behaviour, where we end up doing a DRI2GetBuffers()
call from intelMakeCurrent().  The idea was that we could do this
lazily, just before we start rendering.  However, if we don't do the
DRI2GetBuffers() round-trip we don't get the drawable size and higher
level mesa ends up short-cutting a number of GL calls, such as glClear().

src/mesa/drivers/dri/intel/intel_context.c

@@ -887,6 +887,7 @@ intelMakeCurrent(__DRIcontext * driContextPriv,
       intel->driDrawable = driDrawPriv;
       driContextPriv->dri2.draw_stamp = driDrawPriv->dri2.stamp - 1;
       driContextPriv->dri2.read_stamp = driReadPriv->dri2.stamp - 1;
+      intel_prepare_render(&intel->ctx);
    }
    else {
       _mesa_make_current(NULL, NULL, NULL);