options: add --cache-default option

Add this option, which lets users set the cache size without forcing it
even when playing from the local filesystem.

Also document the default value explicitly.

The Matroska linked segments case is slightly simplified: they can
never come from network (mostly because it'd be insane, and we can't
even list files from network sources), so the cache will never be
enabled automatically.
cocoa_common: change video size when changing screen DPI

This improves the situation when moving a window from screens with different
DPIs.