Hey again! I think I found the problem, and the solution is really obscure - it's definitely an Intel driver bug, but at least this fixes it.
Download this:
http://mp2.dk/shaderfix.zipExtract the 'Data' folder to your 'Knytt Underground' directory, and make sure the shader files are overwritten.
If the problem persists, please tell me - I'm keen to get the bugs fixed. Thanks!