fetchart: Only resize when necessary
fetchart: Only resize when necessary

Fixes #1264: When a re-import triggers fetchart, the existing album art is no
longer resized if it already has the correct dimensions. This avoids creating
new album art files with unique filenames ("cover.n.jpg").

mosesfistos1/beetbox,pkess/beets,mosesfistos1/beetbox,jackwilsdon/beets,Freso/beets,jackwilsdon/beets,jmwatte/beets,SusannaMaria/beets,beetbox/beets,sampsyo/beets,sampsyo/beets,beetbox/beets,artemutin/beets,LordSputnik/beets,ibmibmibm/beets,MyTunesFreeMusic/privacy-policy,Freso/beets,Freso/beets,madmouser1/beets,swt30/beets,jcoady9/beets,jmwatte/beets,artemutin/beets,artemutin/beets,madmouser1/beets,mried/beets,LordSputnik/beets,mosesfistos1/beetbox,lengtche/beets,xsteadfastx/beets,xsteadfastx/beets,jackwilsdon/beets,diego-plan9/beets,m-urban/beets,swt30/beets,xsteadfastx/beets,beetbox/beets,ibmibmibm/beets,jmwatte/beets,kelvinhammond/beets,LordSputnik/beets,jcoady9/beets,kelvinhammond/beets,Kraymer/beets,ibmibmibm/beets,lengtche/beets,LordSputnik/beets,artemutin/beets,kelvinhammond/beets,sampsyo/beets,shamangeorge/beets,pkess/beets,shamangeorge/beets,jcoady9/beets,MyTunesFreeMusic/privacy-policy,kareemallen/beets,parapente/beets,jcoady9/beets,mosesfistos1/beetbox,jmwatte/beets,shamangeorge/beets,madmouser1/beets,MyTunesFreeMusic/privacy-policy,jackwilsdon/beets,shamangeorge/beets,diego-plan9/beets,parapente/beets,sampsyo/beets,pkess/beets,m-urban/beets,swt30/beets,diego-plan9/beets,Kraymer/beets,ibmibmibm/beets,lengtche/beets,SusannaMaria/beets,xsteadfastx/beets,m-urban/beets,Kraymer/beets,mried/beets,diego-plan9/beets,madmouser1/beets,kareemallen/beets,kareemallen/beets,MyTunesFreeMusic/privacy-policy,m-urban/beets,pkess/beets,lengtche/beets,parapente/beets,kareemallen/beets,mried/beets,mried/beets,Kraymer/beets,parapente/beets,beetbox/beets,SusannaMaria/beets,Freso/beets,SusannaMaria/beets,kelvinhammond/beets,swt30/beets