This appears to fix gregorio-project#1458.  Please test on other systems.

There was a whole bunch of repeated code on both branches of several if statements.  I've pulled that code out to reduce the if branches down to the absolute minimum.

The gre@biginitial flag is redundant.  The same information can be found out by checking the value of gre@count@initiallines.

I don't know what circumstances led to this backup being put into place, but I the whole test suite passes without it, so I don't think it's necessary anymore.

We want to identify gre@knownline as a count
Also adding in debug messages associated with knownline to help with the identification of when lines are processed (eventually so we can adjust the staff lines at the right places).

initialraise is already accounted for when \gre@dimen@temp@five is first established (line 402).  It doesn't need to be double counted for the large initial.

This makes the height placement of the initial a function of the number of lines which wrap it rather than a fixed amount.  It doesn't change anything for the 1 or 2 line initials, just makes it so that when 3 (or more) line initials are implemented, this calculation will be ready for them.

As staff lines with fewer or more than 4 lines have different heights so the position of the initial needs to be adapted as appropriate.  This change fixes the new tests introduced in gregorio-project/gregorio-test@8713c85 (and amended in gregorio-project/gregorio-test@d8829fc).

We already calculate the staffheight and don't need to calculate it again here.
Note: making this change reveals an off-by-one error in the manual calculation: there was an extra interstafflinespace.  The number of spaces between the lines is one less than the number of lines: line-space-line

In line with 8106316 these are calculations which should have been using staffheight but weren't.
Also found another off-by-one error in the placement of the above lines text.  This was hidden by the fact that the size of the lyrics wasn't being taken into account.
The change in gregoriotex-signs.tex did not have an off-by-one error because we're drawing the ledger line exactly one interstafflinespace above the staff.  Hence, when we change to using staffheight, we still need one more factor of interstafflinespace

A pretty basic simplification of a calculation.

In semi-manual mode, only the last line of the lines wrapping the initial needs to be indicated manually (as opposed to the manual mode, where each line must be indicated).

Accidentally removed this when merging other branches in.

Since there are only two possible states, the code can be simplified if this flag is a boolean.

…with multi-line initials