Fix for Issue #441: gl-disabled-vertex-attrib.html too strict

Move the test from the fragment shader to the vertex shader to avoid relying on the accuracy of the perspective correction implementation.
Fix for Issue #441: gl-disabled-vertex-attrib.html too strict

Move the test from the fragment shader to the vertex shader to avoid relying on the accuracy of the perspective correction implementation.
Merge pull request #445 from c0d1f1ed/fix-gl-disabled-vertex-attrib

Fix for Issue #441: gl-disabled-vertex-attrib.html too strict