changed: allow cache buffer size to be configured via advancedsettings rather than hardcoded.

Thanks ralfonat. Acked by elupus.

(cherry picked from commit acf0e0874177f9b44259d06528f61e7959a375d6)

git-svn-id: https://xbmc.svn.sourceforge.net/svnroot/xbmc/branches/Dharma@35643 568bbfeb-2a22-0410-94d2-cc84cf5bfa90