sixel: calculate default bg once, in init

We have all information we need to calculate the default background
color in sixel_init():

* Whether the image have transparency or not
* The current ANSI background color

terminal.h

@@ -576,6 +576,7 @@ struct terminal {
         unsigned param_idx;  /* Parameters seen */
 
         bool transparent_bg;
+        uint32_t default_bg;
 
         /* Application configurable */
         unsigned palette_size;  /* Number of colors in palette */