Adjust spacelinestext

The value of `spacelinestext` (distance from the lines to the baseline of the lyrics) was previously a fixed value, which meant that especially large lyrics would overlap with low notes (and small lyrics would seem very far from the lines).  By changing it to a font based value, the lyrics should naturally adjust their position based on their size.  The converted value corresponds to keeping the lyrics in the same place for 11pt Alegrya (the test font), as this causes the fewest tests to change.

CHANGELOG.md

@@ -5,6 +5,7 @@ As of v3.0.0 this project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased][develop]
 ### Fixed
 - Fixed some problem in 900_gregorio.xml (Scribus render frame tool).  First, the use of `filecontents` rather than `filecontents*` was leading to a comment header that made it impossible for Gregorio to find the gabc headers in the temporary score file.  Further, some of the indenting (which makes the file more human readable) was leading to errors in the formatting of the created files because they are processed in a way which handles whitespace differently from XML.  See [#1457](https://github.com/gregorio-project/gregorio/issues/1457).
+- `spacelinestext` is now based on the lyric font size instead of being a fixed distance.  As a result, large lyrics should no longer overlap with low notes.  See [#1261](https://github.com/gregorio-project/gregorio/issues/1261).
 
 ### Added
 - Default spacings are now designated internal and thus always loaded.  `gsp-sample.tex` is added to the `doc` folder to show users how to create their own custom spacing configuration.  As part of this change, spacing configuration files no longer need to be complete.  Since the default configuration is always loaded at package startup, all needed penalties and spacings will be defined and the user's configuration file need only specify those whose value they wish to customize.  Addresses issues raised in [#1460](https://github.com/gregorio-project/gregorio/issues/1460).  **This is a change to the user interface and warrants a major release.**

tex/gsp-default.tex

@@ -361,7 +361,7 @@
 % same, for notes taken into account for nabc vertical position
 \grechangecount{additionaltopspacenabcthreshold}{4}%
 %the space between the lines and the bottom of the text
-\gre@createdim{spacelinestext}{0.60617 cm}{scalable}%
+\gre@createdim{spacelinestext}{3.48471ex}{fixed}%
 %the per-note additional space between lines and the bottom of the text
 \gre@createdim{noteadditionalspacelinestext}{0.14413 cm}{scalable}%
 % this counter is the number of low notes which will add on the