Avoid measuring the length of strings repeatedly by passing their
length along with their data with `StringRefNull`. Null termination
seems to be necessary still for passing the shader sources to OpenGL.
Though I doubt this is a bottleneck, it's still nice to avoid overhead from
string operations and this helps move in that direction.
Pull Request: https://projects.blender.org/blender/blender/pulls/127702
9b97ba1462