glut mouse position off (plus possible fix)

[Floh]

Hi,

I'm getting wrong absolute mouse positions in the Y axis from library_glut.js, I think I only noticed this now because I added fullscreen mode recently, and the mouse offset was very noticeably off there. library_sdl.js uses a different approach to compute mouse position (using getBoundingClientRect) and I tried the same in library_glut.js and it worked. library_glfw.js also seems to be affected from looking at the code (looks like it has been copied from library_glut.js).

I wrote a new issue with more details here: https://github.com/kripken/emscripten/issues/1169

Cheers,

-Floh.