meson: fix automagic dependency lookup for cairo

Using custom required: conditions based on msvc fallbacks means that cairo can be found whenever it is installed, even when disabled. This breaks logic checks later on, and produces wasteful lookups when force disabled anyways.

Fix this by using the same logic meson would use with a true feature option, and cause cairo to always be not-found when it is disabled, using a guaranteed blank / not-found dependency.