I have an older 2012 iMac which works fine, except it's deadly slow with a lethargic spinning 1 terrabyte drive. If I pickup an SSD equipped mac Mini and use the iMac as a display, what GPU will it be using? The iMac's or the mac mini's? Would I still be able to access any files on my iMac or does it simply become just a dumb monitor? Thanks.