skipExportGlyphs should be later in the pipeline

I suggest that `skipExportGlyphs` should be processed later in the pipeline, at least later than `flattenComponents` (which is already after `decomposeComponents`). The current implementation results in unnecessary font bloat.

Consider this common situation: an encoded composite glyph references a component glyph that is itself a composite. Here is a concrete example (with non-relevant detail omitted) from Arabic, but it happens in Latin and other scripts as well:
```

...

```
We have `_dot2h` used as a component (of `qaf-ar`) but is itself a composite glyph consisting of two references to `_dot1`.

GoogleFonts team requires this to be flattened, so we include the `flattenComponents` filter which will then flatten the `qaf-ar` glyph. The resulting glyph, if represented in UFO-speak, would be:
```

```

This was confirmed by building the TTF and dumping with ttx, which yields:
```





```

Now, of course, the `_dot2h` is no longer used — it is not encoded and all other composites referencing it would similarly have been flattened. As a result, fontBakery legitimately complains that the `_dot2h` glyph is unreachable.

It would be nice to now be able to utilize the `skipExportGlyphs` mechanism to remove the unreachable `_dot2h` from the resulting font and quiet the fontBakery warning.

**However**, since ufo2ft processes `skipExportGlyphs` _first_, and at this point `qaf-ar` still references `_dot2h`, adding `_dot2h` to `public.skipExportGlyphs` will cause the `qaf-ar` glyph to be [needlessly] fully decomposed into a contour glyph.

Multiplying this over all the glyphs in the font that get similarly decomposed results in a significant increase in the glyph table size. In the [Lateef project](https://github.com/silnrsi/font-lateef), adding just `_dot2h` to `public.skipExportGlyphs` increased the glyph table by 14 kb.

If the `skipExportGlyphs` process were run later in the pipeline, at least after `flattenComponents`, then it could have the desired effect of making the font smaller.

Trace the ufo2ft pipeline around skipExportGlyphs, decomposeComponents, and flattenComponents, then reproduce the Lateef-style composite-glyph case described in the issue. Build a TTF and inspect the resulting glyph table with ttx; done means skipExportGlyphs runs after flattenComponents without unnecessarily decomposing affected composites or leaving unreachable glyphs.

Written by the indexing model from the issue text.