Add a new padding template format option

[geographika]

See #5889 - adds a new "padding" format option for fixed length strings in a template.

Online testing in REPL at https://repl.it/repls/LeftSlimQuotient
Reviews of C code welcome, especially if variable declarations should be moved, and everything is created and freed correctly.

[rouault]

Would be good to extend (or create) a msautotest for this

[geographika]

@rouault - many thanks for the review and explanations. I'll try and find an autotest for templates to add this to (also I'll update docs if this pull request is accepted).

[sdlime]

I think I’d just ignore the value. Is this for only padding on the right? What about supporting padding on the left or both? A negative number would be convenient for left padding, no idea on both. You’d almost need a second attribute, padding direction… Then again I don’t understand the use case very well.

[geographika]

@sdlime - the main use case is to allow for creation of RST tables which rely on fixed width strings. I've no current use case for left padding though.
I've added docs and the RST table example to MapServer/MapServer-documentation#286
@rouault - thanks for taking the time to explain the errors. I think this is my first non-Python MapServer "feature" :)

[rouault]

@rouault - thanks for taking the time to explain the errors. I think this is my first non-Python MapServer "feature" :)

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