graphics/text: set -colorize alpha channel blend percentage to zero

Give a fourth channel blend percentage of zero so that the alpha channel
of the default fill colour (fully opaque black) is ignored. This fixes
the blue menu titles (skill, episode etc.) having black backgrounds.

The use of -colorize here blends 100% of the default fill colour (black)
into one or more channels of a greyscale image. Mixing in 100% black is
equivalent to nullifying the channel by setting it to zero, e.g. red
text is achieved by mixing 100% black into the green and blue channels.

It turns out, when passed more than one value (to affect all channels by
that percentage equally), -colorize looks for _four_ separate channel
values (RGB and alpha). If fewer are given, the missing ones inherit the
blend percentage of the first channel (red).

For red and white there is no effect, as the red channel is left alone.
But blue blends 100% of the red channel (thus, unwittingly, the alpha
channel) of fully opaque black into the image, making it fully opaque...