gl_FragCoord.xy test

If no multisampling : gl_FragCoord.xy should always be exact  at the
center of the framebuffer pixel.
Pass :
Chrome 35.0.1916.153 m
37.0.2043.0 canary
Opera 22.0.1471.50
Firefox 29
Maxthon v4.3.2.1000

Fail :
IE11 11.0.9600.17126