[SDL] Actually fix SDL_UnlockSurface in IE10/IE11.

The previous revision did not copy over pixel data properly. This time, I have reverted the IE path to the code used in this method prior to the breaking change, and have tested it with JSMESS.
Merge pull request #1973 from jvilk/sdl_fix

[SDL] Actually fix SDL_UnlockSurface in IE10/IE11.