vcl: make MapMode constructor explicit

Insert constructor everywhere, except a couple places that apparently
want to compare GetMapUnit().

Change-Id: I1910deb60562e5e949203435e827057f70a3f988

cppcanvas/source/mtfrenderer/implrenderer.cxx

@@ -805,7 +805,7 @@ namespace cppcanvas
                 rFontSizeLog = ::Size(0, 16);
 
                 // convert to target MapUnit if not pixels
-                rFontSizeLog = OutputDevice::LogicToLogic(rFontSizeLog, MapUnit::MapPixel, rParms.mrVDev.GetMapMode());
+                rFontSizeLog = OutputDevice::LogicToLogic(rFontSizeLog, MapMode(MapUnit::MapPixel), rParms.mrVDev.GetMapMode());
             }
 
             const sal_Int32 nFontWidthLog = rFontSizeLog.Width();