ligo-skymap-plot-volume does not respect transparent flag for the entire png

When creating volume renderings of sky maps using ligo-skymap-plot-volume, the --transparent flag is off by default. This is only respected for the subplot of the distance marginalization posterior, instead of the entire figure. This makes it difficult (or impossible) to read the axes labels (see, e.g., emfollow/gwcelery#752 (comment 927465)).

Here's an example of two plots made of the bayestar skymap from https://gracedb.ligo.org/superevents/S240109a/view/.

Non-transparent plot made with

ligo-skymap-plot-volume bayestar.multiorder.fits -o plot_vol_no_transparent.png

Transparent plot made with

ligo-skymap-plot-volume bayestar.multiorder.fits -o plot_vol_transparent.png --transparent

I think the intended behaviour should be to make the background of the entire figure (i.e., including all subplots) white if the transparent flag is not turned on, so that the labels are readable in all situations.