make FileName::isZipped more efficient by caching previous results
* src/insets/insetgraphics.C
(copyToDirIfNeeded): replace file_in and zipped arguments with a
FileName argument
(InsetGraphics::prepareFile): adjust call of copyToDirIfNeeded
* src/support/filename.C
(FileName::FileName): set zipped_valid_
(FileName::set): ditto
(FileName::erase): ditto
(isZipped): use zipped_
* src/support/filename.[Ch]
(zipped_): new cache for isZipped()
(zipped_valid_): new, tell whether zipped_ is valid
git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@14376
a592a061-630c-0410-9148-
cb99ea01b6c8