Comment # 10 on bug 63579 from Erik Faye-Lund

Another data-point: My AMD driver also does not support line continuation
characters by default, although they do *after* a "#version 420" line.

So in conclusion, I think blindly supporting line-continuation without a
"#version 420"-line is both in violation of the spec as well as diverging from
other implementations in a way that can only create problems.