make core vips deps recommended

libvips has a lot of dependencies: some are used to implement core
functions, some are used to add support for less-used file formats.
Previously all non-required dependencies were :optional, so, by default,
libvips was missing a lot of features.

This patch tags dependencies which are used to implement vips operations
and support for common file formats as :recommended and leaves those
which are used for less-common file formats as :optional.