CSS not always added by chessboard_init() for cacheable formats [regression]

Introduced in 6.x-1.x-rc3. See #922060: Do explicit check if format is cachable.

The loop in chessboard_init() stops executing at the first non-cacheable format.
To reproduce:
-add the filter to both a cacheable and non-cacheable format and arrange for the non-cacheable format to be retrieved before any cacheable format with the chessboard filter.
-visit a page that has no chessboards in the non-cacheable format.