Unifying code for drawing on a cairo context

[Pfrommer, Julius]

The X and the PGTK toolkits both support a cairo "glass". Their code for
drawing on a cairo context is virtually identical. Yet the
implementations have started to diverge.

As an example, x_draw_horizontal_wave and pgtk_draw_horizontal_wave are
virtually the same. But the X implementation takes a "GC" graphics
context, whereas the PGTK implementation now takes a "color" integer
argument.

https://git.savannah.gnu.org/cgit/emacs.git/tree/src/xterm.c#n5155
https://git.savannah.gnu.org/cgit/emacs.git/tree/src/pgtkterm.c#n1995

Obviously maintaining duplicate code is an anti-pattern. I'd like to
pull out common cairo functionality bit-by-bit into methods to be used
by both the X and the PGTK toolkit.

With the cairo primitives unified between X and PGTK, adding a cairo
"glass" for more toolkits should also become easier as a future option
(ns, haiku, w32).

But I'm unsure about how this fits with ongoing development and which
conventions to follow.

- Would the unified cairo function be called cairo_draw_horizontal_wave,
establishing a new cairo_ prefix convention?
- Should such functions generally take an immediate cairo_t context as
input or some "larger" struct that includes that cairo_t and can be
extended in the future (e.g. with foreground and background color
information).
- Is it okay to create the files cairoterm.h/.c or can we expect
functionality beyond "term" to be added over time as well?

Regards,
Julius