Make fthbfont and derivatives use common HarfBuzz code in hbfont.c

* src/font.h (fthbfont_shape, fthbfont_combining_capability)
[HAVE_HARFBUZZ]: Remove prototypes.

* src/ftfont.c: Don't include math.h.
(uni_combining, uni_general, uni_mirroring, get_hb_unicode_funcs)
(fthbfont_shape_by_hb, fthbfont_combining_capability, fthbfont_shape)
[HAVE_HARFBUZZ]: Remove functions.

* src/ftfont.c (syms_of_ftfont_for_pdumper) [HAVE_HARFBUZZ]:
* src/xftfont.c (syms_of_xftfont_for_pdumper) [HAVE_HARFBUZZ]:
* src/ftcrfont.c (syms_of_ftcrfont_for_pdumper) [HAVE_HARFBUZZ]: Use
hbfont_shape and hbfont_combining_capability for fthbfont_shape and
fthbfont_combining_capability, respectively.
1 job for harfbuzz in 3 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #1982


Name Stage Failure
test-all Test There has been a runner system failure, please try again
Running with gitlab-runner 11.6.0 (f100a208)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:8d31923452f8b79ae91b01568d28c90e7d667a9eaff9734c6faeb017b0efa8d0 for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: failed to start shim: exec: "docker-containerd-shim": executable file not found in $PATH: unknown (executor_docker.go:1018:0s)