Cogl sets this for us since commit 2701b93f159bf2d3387cedf2d06fe921ad5641f3.
Setting it twice is illegal and causes compile failures:
error C0204: version directive must be first statement and may not be repeated.
This doesn't (or shouldn't) change the visual appearance of the fade
effect, but does do all the testing math inside the shader, rather
than on the CPU. This will make fading the offset much easier in
the future.
https://bugzilla.gnome.org/show_bug.cgi?id=689249
GLSL 1.20 is a better language, and we'll rely on it in future updates.
This doesn't have any additional constraints, since GLSL 1.20 was
standardized before GLSL-supporting drivers came out.
https://bugzilla.gnome.org/show_bug.cgi?id=689249
