Add customizable background color options for 'bitmap' exporting

Summary:

When saving a JPG image, Inkscape is saving it with a black background.
NOTE: using fix from inkscape#2860 (comment 701062281) to be able to export JPGs.

Steps to reproduce:

1. open: drawing.svg
2. Shift+Ctrl+E to activate export.
3. change export type to JPG (either via 'Export As', or manually changing the file name)
4. Export

What happened?

JPG is exported with a Black background.

What should have happened?

JPG should have been exported using the 'Background color' that is in Document Properties instead since JPG can't do opacity unlike PNG.

Version info

Inkscape 1.1.1 (3bf5ae0d25, 2021-09-20) x64
Windows 7 x64 SP1