Partially revert "Test that setting line width to NaN is not an error"

This turned out to be not a driver bug after all, but within allowed
behavior. GL specs say that providing NaN or infinity to a function
accepting floating point data yields unspecified results (GLES3.0 2.1.1).
So actually, generating an error is allowed in this case.

Keep the part of the commit that fixed a comment in the test file.

This reverts commit cf16a1cae5179c76397e7bb6f56e6e91dd0cf650.
Merge pull request #661 from NVIDIA/line-width-nan-revert

Partially revert "Test that setting line width to NaN is not an error"
Specify an error for setting line width to NaN

OpenGL drivers are allowed to generate errors when NaN is supplied to
functions under GLES 3.1 section 2.3.3.1 or OpenGL 4.5 section 2.3.4.1.
Similar clauses exist in earlier versions of the specs as well. NVIDIA
OpenGL driver generates an error when NaN is supplied to glLineWidth.
Make this behavior consistent for WebGL, which removes the possibility of
drawing with an undefined line width.

Functions which supply data to shaders are not changed, as this would
have an undesirable performance cost and only a small benefit, since it
would not prevent shaders themselves to generate NaNs.
Merge pull request #662 from NVIDIA/line-width-spec

Specify an error for setting line width to NaN
Add missing short explanations buffer and framebuffer functions

Also fixes some small mistakes, such as slightly wrong spec section
numbers. The name of the parameter of ReadBuffer in the GLES spec is
"src", the man pages erroneously have "mode".
Merge pull request #663 from NVIDIA/webgl-buffer-spec

Add missing short explanations for buffer and framebuffer functions