Re: Add ly:get-font-format to get the font format (issue 296350043 by address@hidden)

[trueroad]

> LGTM, but note that FreeType's `FT_Get_Font_Format` doesn't really
return the
> font format but the name of the module used to handle a font.  In
particular, it
> doesn't make a difference between PFA and PFB.

My main purpose was the distinction between the TTC and OTC.
So this patch cannot distinguish between the PFA and PFB.

> Such a function could be then used to rewrite `font-file-as-ps-string`
to avoid
> any dependency on the file extension.

I'm trying the following `font-file-as-ps-string' with this Patch Set 2.

```
  (define (font-file-as-ps-string name file-name font-index)
    (let* ((downcase-file-name (string-downcase file-name)))
      (cond
       ((and file-name (string-endswith downcase-file-name ".pfa"))
        (embed-document file-name))
       ((and file-name (string-endswith downcase-file-name ".pfb"))
        (ly:pfb->pfa file-name))
       ((and file-name (string-endswith downcase-file-name ".ttf"))
        (ly:ttf->pfa file-name))
       ((and file-name (string-endswith downcase-file-name ".ttc"))
        ;; TODO: distinguish files which have extension `*.ttc'
        ;; whether TrueType Collection (TTC) fonts
        ;; or OpenType/CFF Collection (OTC) fonts.
        (ly:ttf->pfa file-name font-index)) ;; TTC fonts
       ((and file-name (string-endswith downcase-file-name ".otf"))
        (ps-embed-cff (ly:otf->cff file-name) name 0))
       ((and file-name (string-endswith downcase-file-name ".otc"))
        ;; The files which have the extension `*.otc' are OTC fonts.
        (ps-embed-cff (ly:otf->cff file-name font-index) name 0)) ;; OTC
fonts
       (else
        (ly:warning (_ "do not know how to embed ~S=~S") name file-name)
        ""))))
```

> Wouldn't it be better to return a symbol instead?

I've uploaded Patch Set 2 that uses a symbol instead of a string.

https://codereview.appspot.com/296350043/