support sparse sources workflow

[gferreira]

proposal to adapt measurement tools in order to support a workflow using sparse sources.

1. add a fallback font for missing measurements

when a sparse source does not include a glyph which is needed for a measurement, a fallback font (the default) must be used. measurement tools need a new function to define this font.

2. remove attribute glyph2 from font-level measurements (optional)

cross-glyph measurements should no longer be allowed; both points involved in a measurement must come from the same glyph, as it is with glyph-level measurements. named points are also supported for xheight, capheight, origin point, etc.

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see also TempGlyphs, a new tool to ‘fill’ sparse sources with glyphs from another font, and clear them when done.