New function is added.
/** use only one layer to render. caching adjacent pages will be disabled. */
// for low spec devices such as iPad 2 or the epub which has very large size images (the max size of width or height exceeds 1024) in it, [fv useSingleLayer] function enforces skyepub sdk to use only one webView layer to reduce the consumed memory.
// this function works only with NONE,SLIDE mode,