workspace: Don't redraw workspace if it doesn't need redrawing

workspace: Don't redraw workspace if it doesn't need redrawing

The mwm_redraw() function is called to redraw the clients as well as
the status bar and other indicators. The current implementation calls
monitor_redraw() and workspace_redraw() on all monitors and workspaces,
respectively, so these functions must make sure not to redraw anything
if it isn't necessary.
This commit changes the workspace_redraw() function so it does not
redraw the workspace if it was not explicitly requested.