Implement the get_variation_glyphs method for HarfBuzz on MS-Windows

* src/w32uniscribe.c [HAVE_HARFBUZZ]: DEF_DLL_FN
(hb_font_get_variation_glyph): New redirection macro.
(w32hb_get_variation_glyphs): New function.
(load_harfbuzz_funcs): Load hb_font_get_variation_glyph.
(syms_of_w32uniscribe_for_pdumper): Populate the
get_variation_glyphs method of harfbuzz_font_driver.
1 job for harfbuzz in 19 seconds (queued for 4 seconds)
Status Job ID Name Coverage
failed #1966


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:1018:10s)