The conversion from a known path prefix to a variable is now done in a
case-insensitive way. This means that if the variable `{storage}` has
value `S:\Flamenco`, a path `s:\flamenco\project\file.blend` will be
recognised and replaced with `{storage}\project\file.blend`.
This happens uniformly, regardless of the platform. So also on Linux,
which has a case-sensitive filesystem, this matching is done in a
case-insensitive way. It is very unlikely that a Flamenco configuration
has two separate variables, for paths that only differ in their case.
Fixes: #104336 Drive letter case mismatch causes two way variables not
to work correctly
When expanding the `{shared}` variable in a path like
`{shared}\shot\file.blend`, all slashes will be normalised to the target
platform, so for example result in:
- Windows: `Y:\shared\flamenco\shot\file.blend`
- Linux : `/shared/flamenco/shot/file.blend`
Due to this normalisation, the same paths will be served to these
platforms when the path was `{shared}/shot/file.blend`.
Simplify the code for the two-way variables' value-to-variable replacement.
Instead of using a goroutine and two channels, use a separate struct and
call a function on that directly.
No functional changes.
