filter: use build tags to separate debug code from release code

[Saxon Milton]

We’ve got a lot of debug code in our filter files. We should have debug tags defining debug code, and then !debug tags for ‘release’ code. See here for example.

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[Scott Barnard]

If we have debug tags for building, should we also remove the ShowWindows variable, which is used for enabling and disabling the display of the windows?

[Scott Barnard]

I’ll look at this. @Scott Barnard