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.