Use component library symbol when looking for a pin in the schematic.

For some reason, rather than using the LIB_PART object reference by an
SCH_COMPONENT to check if a pin was at a given position in the schematic,
a LIB_PART search was performed for every component in a schematic.  This
made absolutely no sense and was changed to use the LIB_PART referenced
by the SCH_COMPONENT object.  This should significantly speed up any
operations that call SCH_SCREEN functions, IsTerminalPoint(),
MarkConnections(), and IsJunctionNeeded().

Add function to SCH_COMPONENT to access PART_REF object.

eeschema/sch_component.h

@@ -164,6 +164,8 @@ public:
     void SetLibId( const LIB_ID& aName, PART_LIBS* aLibs=NULL );
     const LIB_ID& GetLibId() const        { return m_lib_id; }
 
+    PART_REF& GetPartRef() { return m_part; }
+
     /**
      * Function Resolve
      * [re-]assigns the current LIB_PART from aLibs which this component

eeschema/sch_screen.cpp

@@ -550,9 +550,6 @@ void SCH_SCREEN::CheckComponentsToPartsLinks()
             SCH_COMPONENT::ResolveAll( c, libs );
 
             m_modification_sync = mod_hash;     // note the last mod_hash
-
-            // guard against unneeded runs through this code path by printing trace
-            DBG(printf("%s: resync-ing %s\n", __func__, TO_UTF8( GetFileName() ) );)
         }
     }
 }
@@ -646,7 +643,7 @@ LIB_PIN* SCH_SCREEN::GetPin( const wxPoint& aPosition, SCH_COMPONENT** aComponen
         {
             pin = NULL;
 
-            LIB_PART* part = Prj().SchLibs()->FindLibPart( component->GetLibId() );
+            auto part = component->GetPartRef().lock();
 
             if( !part )
                 continue;