Support building Glyphs corners with anchors

https://glyphsapp.com/learn/reusing-shapes-corner-components

```
INFO:fontmake.font_project:Building master UFOs and designspace from Glyphs source
INFO:glyphsLib.parser:Parsing .glyphs file
INFO:glyphsLib.builder:Running 'align_alternate_layers' transformation
INFO:glyphsLib.builder:Running 'propagate_all_anchors' transformation
INFO:fontmake.font_project:Loading 9 DesignSpace source UFOs
INFO:fontmake.font_project:Building OTF for TestFontAlternateGlyphs-Ultra-condensedHairline
INFO:ufo2ft:Pre-processing glyphs
INFO:ufo2ft.filters:Running CornerComponentsFilter on TestFontAlternateGlyphs-Ultra-condensedHairline
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H)
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H)
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H)
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H)
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H)
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H)
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H)
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H)
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H.condensed)
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H.condensed)
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H.condensed)
ERROR:glyphsLib.filters.cornerComponents:left and right anchors to corner components are not currently supported (corner _corner.serif in H.condensed)
INFO:ufo2ft.filters:Running EraseOpenCornersFilter on TestFontAlternateGlyphs-Ultra-condensedHairline
INFO:ufo2ft.filters:Running DecomposeComponentsFilter on TestFontAlternateGlyphs-Ultra-condensedHairline
INFO:ufo2ft.filters:Running RemoveOverlapsFilter on TestFontAlternateGlyphs-Ultra-condensedHairline
ERROR:ufo2ft.filters.removeOverlaps:Failed to remove overlaps for _corner.serif
fontmake: Error: In 'TestFontAlternateGlyphs.glyphspackage' -> 'master_ufo/TestFontAlternateGlyphs.designspace': Compiling UFO failed: Unhandled open contour
```

[TestFontAlternateGlyphs.glyphspackage.zip](https://github.com/user-attachments/files/24148777/TestFontAlternateGlyphs.glyphspackage.zip)

Reproduce the failure with TestFontAlternateGlyphs.glyphspackage.zip and trace glyphsLib.filters.cornerComponents alongside ufo2ft's CornerComponentsFilter. Investigate how left and right anchors are handled for corner components, then verify that the supplied Glyphs package compiles without the reported unsupported-anchor and unhandled-open-contour errors.

Written by the indexing model from the issue text.