Fix headup hole lookup (#267) (#268)

When searching for a particular headup slot, don't return on the first
empty hole, but rather try to find if the matching slot is in use.

This resolves a bug where two headups of the same type could end up
displayed on top of each other.

Signed-off-by: Artur Rojek <contact@artur-rojek.eu>

src/DETHRACE/common/displays.c

@@ -618,7 +618,6 @@ int FindAHeadupHoleWoofBarkSoundsABitRude(int pSlot_index) {
         }
         if (the_headup->type == eHeadup_unused) {
             empty_one = i;
-            break;
         }
     }
     return empty_one;