Meson: Change png/jpeg/tiff options from boolean to feature

png/jpeg are essential and have a fallback subproject so they are enabled by default. tiff is not required by GTK and does not have a subproject so it's set to 'auto' by default.

This fixes the case where tiff option was set to true by default but meson setup was not aborting if the dependency was not found. Instead it was failing at build time.