Set the Specification Revision field to 2.0 or 3.0

Based on the first message sent from the Source, we now set the
Specification Revision field of our messages to 2.0 or 3.0.  We don't do
anything else 3.0 yet though, so it's a bit of lie, but this is a
necessary first step.

lib/src/policy_engine.c

             /* If we got a Source_Capabilities message, read it. */
             if (PD_MSGTYPE_GET(cfg->pe._message) == PD_MSGTYPE_SOURCE_CAPABILITIES
                     && PD_NUMOBJ_GET(cfg->pe._message) > 0) {
+                /* First, determine what PD revision we're using */
+                if ((cfg->pe.hdr_template & PD_HDR_SPECREV) == PD_SPECREV_1_0) {
+                    /* If the other end is using at least version 3.0, we'll
+                     * use version 3.0. */
+                    if ((cfg->pe._message->hdr & PD_HDR_SPECREV) >= PD_SPECREV_3_0) {
+                        cfg->pe.hdr_template |= PD_SPECREV_3_0;
+                    /* Otherwise, use 2.0.  Don't worry about the 1.0 case
+                     * because we don't have hardware for PD 1.0 signaling. */
+                    } else {
+                        cfg->pe.hdr_template |= PD_SPECREV_2_0;
+                    }
+                }
                 return PESinkEvalCap;
             /* If the message was a Soft_Reset, do the soft reset procedure */
             } else if (PD_MSGTYPE_GET(cfg->pe._message) == PD_MSGTYPE_SOFT_RESET
     /* Initialize the old_tcc_match */
     cfg->pe._old_tcc_match = -1;
     /* Initialize the PD message header template */
-    cfg->pe.hdr_template = PD_DATAROLE_UFP | PD_SPECREV_2_0 | PD_POWERROLE_SINK;
+    cfg->pe.hdr_template = PD_DATAROLE_UFP | PD_POWERROLE_SINK;
 
     while (true) {
         switch (state) {