Fix default Graphics::setcol() value having no alpha

This patch fixes a regression caused by commit
6b1a7ebce6.

When you spawn a crewmate with an invalid color, by doing something like
`createentity(100,100,18,-1,0)` (here the color is -1, which is
invalid), a white crewmate with the color of solid white (255, 255, 255)
would appear.

That is, until AllyTally came along and committed commit
6b1a7ebce6 (Make "[Press ENTER to
return to editor]" fade out after a bit) (PR #158). Then after that
commit, it would seem like the crewmate didn't appear, but in reality
they were just invisible, because they had an invisible color.

As part of Ally's changes, to properly support drawing text with a
certain amount of alpha, she made BlitSurfaceColoured() account for the
alpha of the color given instead of only caring about the RGB of the
color, discarding the alpha, and using the alpha of the surface it was
drawing instead. This effectively made it so the alpha of whatever it
was drawing would be 255 all the time, except for if you had custom
textures and your custom textures had translucent pixels.

However, the default color set by Graphics::setcol() if you didn't
provide a valid color index was 0xFFFFFF. Which is only (255, 255, 255)
but ends up having an alpha value of 0 (because it's actually
0x00FFFFFF). And all colors drawn with alpha 0 end up being drawn with
alpha 0 after 6b1a7ebce6. So
invalid-colored entities will end up being invisible.

To fix this, I just decided to add alpha to the default value instead.
In addition, I used getRGB() to be consistent with all the other colors
in the function.

desktop_version/src/Graphics.cpp

@@ -2717,7 +2717,7 @@ void Graphics::setcol( int t )
 		break;
 
 	default:
-		ct.colour = 0xFFFFFF;
+		ct.colour = getRGB(255, 255, 255);
 		break;
 	}
 }