(enhancement) Get different colormaps to work for background layer in double rendering

[themikelau]

When double rendering, the background layer is hard-coded to be greyscale. But when hard-coded to have a different colormap (just by changing the line icolours_temp = 1 inside if (gotcontours .and. double_rendering), the transparency no longer works.

Related to this issue, it would be nice to have a prompt for the colourmap of the first layer in the double rendering menu, and automatically display the colourbar (maybe half-half on the right when in Hollywood mode).

[themikelau]

@danieljprice Turns out I have filed an issue for the double rendering colour bars. There is also a related issue in the first paragraph. I'm not sure if this is an easy fix.