Pixomondo has released a video reel of Game of Thrones VFX, this one focusing specifically on the process that brought Drogon, Viserion and Rhaegal to life for season 5.
Werner Fuchs (German agent and friend of GRRM) confirmed that GRRM told David and Dan what was going to happen in the next two books in case that he shouldn’t be able to finish the books.
"The dragons mature this season so their look is more fierce, We changed the proportions up a bit and made the spikes more prominent so the dragons are much more menacing now."
This year, due to increased VFX work, these two smaller studios have had to bow out and Thrones producers went searching for a larger VFX studio to take over. It appears they have found their replacement.