integrate line stretching in debug messages

[eroux]

This patch:

diff --git a/tex/gregoriotex.lua b/tex/gregoriotex.lua
index e8b21b7..fe1687f 100644
--- a/tex/gregoriotex.lua
+++ b/tex/gregoriotex.lua
@@ -356,6 +356,8 @@ local function process (h, groupcode, glyphes)
         --log("eating line")
         h, line = remove(h, line)
       else
+        local glue_sign_name = {[0] = 'normal', [1] = 'stretching', [2] = 'shrinking'}
+        texio.write_nl(string.format('new line: %s factor %f', glue_sign_name[line.glue_sign], line.glue_set))
         centerstartnode = nil
         line_id = nil
         line_top = nil

prints debug messages about the stretching/shrinking factors of the different lines. This would be very useful to trigger in the debug interface.