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
Added comment

Added comment
Added newline

Added newline
Fixed indenting

Fixed indenting
Added errorbound to check on pixel center

Added errorbound to check on pixel center.
Do you think it is needed ? I thought about it but decided not to add
the error bound because 0.5 is exact representable in binary (0.1). Even
if there is a gpu which does use an internal representation which can
not exactly represent 0.5 (dec) it should compute a representation which
is the closest it can get for both the fractional coordinate and for the
compiled literal. So the comparison should still succeed. Is there a
central place where I can post these kind of technical questions to
vendors ?
Merge pull request #588 from TiborsHub/master

gl_FragCoord.xy test