In TTML, the height of every line in a block is determined by lineHeight
and fontSize style attributes, and should be the same for each line in
that block, regardless of whether different sized text appears on
different lines. Currently, a single PangoLayout is used to lay out all
the text in a block; however, pango will vary the line height in a
layout depending on the size of text used in each line, which is not
compliant with TTML.
This patch makes ttmlrender lay out the lines in a block itself, rather
than using a PangoLayout to do the work. The code still uses a
PangoLayout to render the text of each element, but the overall layout
of the text in a block is now controlled by ttmlrender itself. By doing
this, ttmlrender is able to ensure that the height of each line in a
block is correct.
https://bugzilla.gnome.org/show_bug.cgi?id=780402
