Change all glyphs to have a transparent background

Done with GIMP. The new files are slightly larger because
of the presence of the alpha channel. Maximum compression
achieved with optipng, advpng and advdef.

Used the following GIMP script-fu procedure (heavily based
on examples from its manual) to remove the white background:

  (define (fix-hiero filename color)
    (let* ((image (car (gimp-file-load RUN-NONINTERACTIVE filename filename)))
           (drawable (car (gimp-image-get-active-layer image))))
    (gimp-image-convert-rgb image)
    (gimp-layer-add-alpha drawable)
    (plug-in-colortoalpha RUN-NONINTERACTIVE image drawable color)
    (gimp-image-convert-grayscale image)
    (gimp-file-save RUN-NONINTERACTIVE image drawable filename filename)
    (gimp-image-delete image)))

  (fix-hiero "hiero_a&A1.png" '(255 255 255)) …

Bug: 31736
Change-Id: Id70fa296d30210364ce9e307d5c51a3ab4875590