loaders: ZIP: Propagate errno from GP_ReadImage()

Propagate errno from GP_ReadImage() from zip_next_file() if loading was
canceled from callback.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: asciidoc.css: underline links on mouse over.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
core: Add GP_ALIGN2() macro.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: Add PSD thumbnail loader

Signed-off-by: Cyril Hrubis <metan@ucw.cz>