This patch modifies 'svg->png' in (gnu system grub) to use rsvg-convert
instead of inkscape.  Inkscape is a rather heavy dependency to convert
an image.  This is important for users who choose to avoid binary
substitutes.

The main difficulty here was that our SVG artwork is partially
transparent, and includes a "Background" layer with a checker-board
pattern.  I guess this layer is for convenience when editing in
Inkscape, and apparently Inkscape excludes the "Background" layer when
exporting to png.  Other tools render all layers.  Therefore, avoiding
Inkscape required code to remove that layer before conversion to png.

Comments and suggestions welcome,

      Mark