Don't use premultiplied alpha in most GLSL generator tests

Many GLSL function conformance tests can generate superluminal color
values, where red, green or blue > alpha. Since browsers don't do
clamping for these color values before they are unpremultiplied and
encoded as PNG when toDataURL is called, the on-screen display of these
tests did not reflect the actual numeric values generated by the shader.
This complicated debugging failed tests. Fix this by switching to a
context with a non-premultiplied drawing buffer in these tests.

This issue does not affect the programmatic comparison of test results,
since that is being done using readPixels.
Revert change to the frozen 1.0.1 suite

It makes sense to fix displaying superluminal values in 1.0.2, since lots
of implementations are still working on conformance, but with 1.0.1 it's
better to avoid this kind of changes.
Merge pull request #312 from NVIDIA/fix-superluminal-display

Don't use premultiplied alpha in most GLSL generator tests