Migrating displays on kmsdrm

[icculus]

Someone DM'd me on Mastodon to say that he could use multiple displays on kmsdrm if he created the window with position properties, but SDL_SetWindowPosition() could not do it.

I wouldn't be surprised if kmsdrm doesn't have SetWindowPosition implemented at all, as there's no window manager there, but it should probably have enough of an implementation to decide a window needs to migrate to a different display.