[SDL] Fixing SDL_UnlockSurface in IE10/IE11.

Previously, just calling SDL_UnlockSurface in IE10/IE11 would throw an exception, since Emscripten assumed that the ImageData's `data` property was Uint8ClampedArray, which has a backing buffer.

IE10/IE11 still uses the deprecated CanvasPixelArray, which does not have a backing buffer property:
https://developer.mozilla.org/en-US/docs/Web/API/CanvasPixelArray

I've added an additional path to SDL_UnlockSurface for these browsers.
Merge pull request #1971 from jvilk/sdl_fixes

[SDL] Fixing SDL_UnlockSurface in IE10/IE11