fix perf bug (I think)

In commit 93160bcb "convert PAINT
constants to typed_flags", I removed two lines of code (based on faulty
analyis), which appears to have caused a performance regression in the
"ILmr" test

Bug noticed and tracked down by Jochen Nitschke, I'm just testing and
committing.

Change-Id: I1c7dfc305765f7b20458421680f776a4a9324281

sc/source/ui/docshell/docsh3.cxx

@@ -126,6 +126,8 @@ void ScDocShell::PostPaint( const ScRangeList& rRanges, PaintPartFlags nPart, sa
             }
 
             nPart &= PaintPartFlags::Extras;  // for broadcasting
+            if (nPart == PaintPartFlags::NONE)
+                continue;
         }
 
         if (nExtFlags & SC_PF_LINES)            // Platz fuer Linien beruecksichtigen