Fix 3D model resolution for older boards

Older boards use older 3d model environmental variables.  We need to
support both older environmental variables (if the user has them) as
well as updated environmental variables (if the user only has a single
3d model path)

This updated the environmental resolver to look first in the named
location and then substitute the new location if the old location is not
defined in the environment

Fixes https://gitlab.com/kicad/code/kicad/issues/10674

common/common.cpp

@@ -27,6 +27,7 @@
 #include <kiplatform/app.h>
 #include <project.h>
 #include <common.h>
+#include <env_vars.h>
 #include <reporter.h>
 #include <macros.h>
 #include <mutex>
@@ -196,6 +197,27 @@ wxString KIwxExpandEnvVars( const wxString& str, const PROJECT* aProject )
                 strResult += tmp;
                 expanded = true;
             }
+            // Replace unmatched older variables with current 3d model locations
+            // If the user has the older model location defined, that will be matched
+            // first above.  But if they do not, this will ensure that their board still
+            // displays correctly
+            else if( strVarName.Contains( "KISYS3DMOD") || strVarName.Matches( "KICAD*_3DMODEL_DIR" ) )
+            {
+                for( auto& var : ENV_VAR::GetPredefinedEnvVars() )
+                {
+                    if( var.Matches( "KICAD*_3DMODEL_DIR" ) )
+                    {
+                        const auto value = ENV_VAR::GetEnvVar<wxString>( var );
+
+                        if( !value )
+                            continue;
+
+                        strResult += *value;
+                        expanded = true;
+                        break;
+                    }
+                }
+            }
             else
             {
                 // variable doesn't exist => don't change anything