DX11: Clean up AccessEFB and fix a crash. Thanks to pico554 for hinting at the wrong part ;)
Should fix SMG for NVIDIA users. Slightly changes behavior of the AccessEFB function, so test this for regressions, please (esp. in HD resolutions).
I tried to apply the same changes to Video_DX9, but for some reason EFB access didn’t work anymore then. Might look into that later.