Use the center of a link's bounding box instead of its upper-left corner to detect if its visible.
Use the center of a link's bounding box instead of its upper-left corner to detect if its visible.

This is a slightly more accurate heuristic and fixes #17.
decaffeinate-examples/vimium,Aleroniponi/vimium,elegantwww/vimium,PrestanceDesign/vimium,crazypenguincode/vimium,poacher2k/vimium,cyrixhero/vimium,elderbas/vimium,wildeyes/vimium,llimllib/vimium,hhsue/vimium,bkudria/vimium,decaffeinate-examples/vimium,wildeyes/vimium,smblott-github/vimium-smblott,VPashkov/vimium,elegantwww/vimium,dimatter/vimium,Robinson10240/vimium,Lw-Cui/vimium,mrmr1993/vimium,tuchangwei/vimium,justalittlenoob/vimium,PickRelated/vimium,wildeyes/vimium,willsALMANJ/vimium,gdh1995/vimium,justalittlenoob/vimium,crazypenguincode/vimium,bkudria/vimium,smblott-github/vimium,csmalin/vimium,poacher2k/vimium,zhangmin510/vimium,justalittlenoob/vimium,willsALMANJ/vimium,hhsue/vimium,gdh1995/vimium,decaffeinate-examples/vimium,philc/vimium,philc/vimium,jb55/vimium-plus,elderbas/vimium,Aleroniponi/vimium,willsALMANJ/vimium,shaung/vimium,jb55/vimium-plus,Lw-Cui/vimium,gdh1995/vimium,douglas-larocca/vimium,elegantwww/vimium,cyrixhero/vimium,douglas-larocca/vimium,borgified/vimium,smblott-github/vimium,PickRelated/vimium,smblott-github/vimium,VPashkov/vimium,csmalin/vimium,tuchangwei/vimium,borgified/vimium,borgified/vimium,PrestanceDesign/vimium,Lw-Cui/vimium,smblott-github/vimium-smblott,dimatter/vimium,dimatter/vimium,Robinson10240/vimium,mrmr1993/vimium,mrmr1993/vimium,PickRelated/vimium,zhangmin510/vimium,hhsue/vimium,philc/vimium,VPashkov/vimium,Aleroniponi/vimium,tuchangwei/vimium,cyrixhero/vimium,zhangmin510/vimium,bkudria/vimium,PrestanceDesign/vimium,poacher2k/vimium,Robinson10240/vimium,csmalin/vimium,elderbas/vimium,douglas-larocca/vimium