window.scrollX/Y is not available in IE11. As far as specifications go, is currently only specified in draft (http://dev.w3.org/csswg/cssom-view/#refsCSSOM). Falling back to 
window.pageXOffset seems like a good workaround.

On a related note; my Emscriptified project runs on IE11 although performance is very poor (mostly due to Internet Explorer itself, I think). It's pretty finicky about the shaders, as they introduced an extra set of requirements. (inout/in/out keywords not supported, can't construct mat3 from mat4, etc).

Added comments to fix as per @kripken's request.
Added an assert too (guarded by ASSERTIONS define)

Forgot I had to explicitly check the type of variable type to see if it's undefined.

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.

[SDL] Fixing 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.

[SDL] Actually fix SDL_UnlockSurface in IE10/IE11.

Added workaround for window.scrollX compat.

clean extra arg of JSify

…avoid confusion with varargs by clang

…fixes