LURE: Original VGA background color in popup menus

[matteobin]

In the original VGA DOS game all GUI dialogs have a dark gray background
color. However in the ScummVM implementation the background color for
popup menus, which are the ones that appear when you right-click, is
black. The ScummVM EGA version works as intended.

Probably this issue was due to the difficulty of knowing at which index
the dark gray color was in all the different palettes. From my
understanding of the LURE engine and VGA, different graphic resources
could have different color palettes.

My solution was to copy the element side lines when the dialog needs to
be emptied, instead of filling a rectangle with solid black. In my
second commit there's even a more strict fix, in which I copy only the
element background, because there is no need to draw the wooden side
borders again.

[dreammaster]

Thanks for taking the time to look into this, much appreciated. We normally recommend users to make a branch for anything they're going to make a PR for, so since yours was done on your own master directly, I've manually applied your commits into our master branch to be on the safe side.