Constant canvas::webgl_thread::ANGLE_NAME_PREFIX

const ANGLE_NAME_PREFIX: &str = "_u";

ANGLE adds a _u prefix to variable names:

https://chromium.googlesource.com/angle/angle/+/855d964bd0d05f6b2cb303f625506cf53d37e94f

To avoid hard-coding this we would need to use the sh::GetAttributes and sh::GetUniforms API to look up the x.name and x.mappedName members.