fdo#79602: sw: fix text formatting of proportional line space < 100%

SwTxtFormatter::CalcRealHeight(): Apply the special treatment of < 100%
line space shrinking the Height() of the line only to the first line
in a paragraph; the subsequent ones are shrunk again (to the square of
the desired proportion) 30 lines later.

Also set the Ascent of the line, as the wrong base-line causes the lower
part of the line to be clipped.

(regression from 42532d42bfcdb12df86376cda009e8e315a82153)

Change-Id: I0424396263293deaa318c80eedc2237cdb01b22b
fdo#79602: sw: add new compatibiltiy flag PropLineSpacingShrinksFirstLine

This is enabled by default, to get the new formatting where the first
line of a paragraph is shrunk if a proportional line spacing < 100% is
applied; existing OOo documents get the previous (before LO 3.3)
formatting.  Since the formatting in LO releases is broken anyway, it
does not matter much which way documents written by old LO get
formatted.

Change-Id: I0952f568a933c137bd03070759989cac3517d8b9