Compute the document before inserting STEP file.

Consider that inserted Tree Element are computed and remove the Touched flag
before recomputing lately the document
This is accelerating the STEP file

src/Mod/Import/Gui/AppImportGuiPy.cpp

@@ -411,7 +411,13 @@ private:
             }
 
             ImportOCAFExt ocaf(hDoc, pcDoc, file.fileNamePure());
+            // We must recompute the doc before loading shapes as they are going to be
+            // inserted into the document and computed at the same time so we are going to
+            // purge the document before recomputing it to clear it and settle it in the proper
+            // way. This is drastically improving STEP rendering time on complex STEP files.
+            pcDoc->recompute();
             ocaf.loadShapes();
+            pcDoc->purgeTouched();
             pcDoc->recompute();
             hApp->Close(hDoc);
         }