D9054 did multiple consecutive `immBegin()`/`immEnd()` draw calls to draw
multiple lines at varying thickness. This would only work for the first line,
then they'd all get a 1px thickness (at least on macOS).
Issue was that `wide_line_workaround_end()` called `immBindShader()` directly
to restore the old shader (which the workaround overrides). However this
doesn't set `imm->builtin_shader_bound` which has to be done for the workaround
to work on the next `immBegin()` call. Instead `immBindBuiltinProgram()` can be
called.
Differential Revision: https://developer.blender.org/D10520
Reviewed by: Clément Foucault
daf7f42364