Fix startvolume parsing in Silverlight fallback

startvolume initvar was improperly parsed using system dependent decimal separator. So if the system setting was to use comma as a decimal separator then startvolume set to e.g. "0.6" (with dot as a separator) was parsed to 0 not to double 0.6 as it should be. As the result player was always muted at start when the volume was set to value between 0-1.

Related to #1323