Fix high cpu usage when minimized and rendering is required

When there is a CGUIDialog shown like OSD or resume dialog and then you minimize the window there is a high cpu usage.
In CGUIDialog::DoModal_Internal there is an infinite loop with basically nothing to do.
Looks like its like this since https://github.com/xbmc/xbmc/commit/872d58c73dcb40f5f7bf71a2d1e4509e676f07e0
Don't know if this is the right way to fix it for xbmc, but it works.