emit a warning if an object instance has alpha transparency on one side but not on the other.

several refactorings and code movements.
added a comment.
More MDF work.
Added simple glass test scene.
Merge pull request #444 from est77/master

More OSL test scenes
updated license notice.
removed old jupiter jazz logos.
Fixed bug when using refraction closure from OSL.
Merge pull request #445 from est77/master

Fixed bug when using refraction closure from OSL.
removed black variants of appleseed logos.
updated appleseed logos (logo pack 1.3).
Merge branch 'master' of github.com:appleseedhq/appleseed
fixed logo pack license.
adjusted appleseed website favicon.
Merge branch 'master' into OSL_microfacet_update
Many changes to MDF refactor classes.
updated favicon and Twitter profile pictures.
Added option to not hide symbols, for old gcc versions.
Small fixes and formatting changes.
Merge pull request #446 from est77/gcc_hide_symbols

gcc hide symbols