Use composition for embedding PangoMatrix in Transform

Use composition for embedding PangoMatrix in Transform

PangoMatrix is a struct, so there is no value in using
inheritance. Use composition instead, so we don't have to use multiple
inheritance.

[hahnjo, 2020-04-13 16:16:17]

https://codereview.appspot.com/555610043/diff/557670044/lily/stencil-integral.cc
File lily/stencil-integral.cc (right):

https://codereview.appspot.com/555610043/diff/557670044/lily/stencil-integral...
lily/stencil-integral.cc:471: 
unrelated change

[hanwenn, 2020-04-13 16:19:23]

jonas

[dak, 2020-04-13 17:25:23]

"PangoMatrix is a struct, so there is no value in using inheritance" is a
non-sequitur.  A struct just is a class with public members.

The motivation for choosing inheritance here was that this allows passing
Transform directly into Pango for operations.  This advantage is not realised in
our current code base, and doing so does not appear like the best long-term plan
since a good long-term plan would likely rather involve using the transform
matrices of Cairo since it would make a lot more sense to route the bulk of
graphics operations through Cairo than maintaining our own limited and
maintenance-intensive set of backends and/or relying on Ghostscript as
comparatively computationally expensive intermediary.

So while I don't see any objective of value achieved here (the objective of not
using multiple inheritance went out the window long ago since it is needed, for
example, for determining initialisation order for GC-marked objects), I don't
see anything adversely impacted, either.