Fixed https://www.khronos.org/bugzilla/show_bug.cgi?id=951 : Incorrect setting of UNPACK_COLORSPACE_CONVERSION_WEBGL in premultiplyalpha-test.html?

Changed setting of parameter from false to gl.NONE.
Merge pull request #356 from kenrussell/colorspace

Fixed https://www.khronos.org/bugzilla/show_bug.cgi?id=951 : Incorrect s...