Automapping: Ignore empty outputs per-rule

This change introduces a "compileOutputSet" step which is executed as
part of collecting the rules. In this step, each rule only receives as
possible outputs the output indexes that are non-empty for the region of
that rule.

This makes it much easier to add output variation to only some of the
rules. Previously, due to the possibility of empty outputs being chosen,
having rules with various amounts of variation required setting up
separate rule maps.

Closes #3523

src/libtiled/tilelayer.h

@@ -251,16 +251,16 @@ class TILEDSHARED_EXPORT TileLayer : public Layer
         {
             if (lhs.mChunkPointer == lhs.mChunkEndPointer || rhs.mChunkPointer == rhs.mChunkEndPointer)
                 return lhs.mChunkPointer == rhs.mChunkPointer;
-            else
-                return lhs.mCellPointer == rhs.mCellPointer;
+
+            return lhs.mCellPointer == rhs.mCellPointer;
         }
 
         friend bool operator!=(const iterator& lhs, const iterator& rhs)
         {
             if (lhs.mChunkPointer == lhs.mChunkEndPointer || rhs.mChunkPointer == rhs.mChunkEndPointer)
                 return lhs.mChunkPointer != rhs.mChunkPointer;
-            else
-                return lhs.mCellPointer != rhs.mCellPointer;
+
+            return lhs.mCellPointer != rhs.mCellPointer;
         }
 
         Cell &value() const { return *mCellPointer; }
@@ -307,16 +307,16 @@ class TILEDSHARED_EXPORT TileLayer : public Layer
         {
             if (lhs.mChunkPointer == lhs.mChunkEndPointer || rhs.mChunkPointer == rhs.mChunkEndPointer)
                 return lhs.mChunkPointer == rhs.mChunkPointer;
-            else
-                return lhs.mCellPointer == rhs.mCellPointer;
+
+            return lhs.mCellPointer == rhs.mCellPointer;
         }
 
         friend bool operator!=(const const_iterator& lhs, const const_iterator& rhs)
         {
             if (lhs.mChunkPointer == lhs.mChunkEndPointer || rhs.mChunkPointer == rhs.mChunkEndPointer)
                 return lhs.mChunkPointer != rhs.mChunkPointer;
-            else
-                return lhs.mCellPointer != rhs.mCellPointer;
+
+            return lhs.mCellPointer != rhs.mCellPointer;
         }
 
         const Cell &value() const { return *mCellPointer; }
@@ -646,8 +646,8 @@ inline const Cell &TileLayer::cellAt(int x, int y) const
 {
     if (const Chunk *chunk = findChunk(x, y))
         return chunk->cellAt(x & CHUNK_MASK, y & CHUNK_MASK);
-    else
-        return Cell::empty;
+
+    return Cell::empty;
 }
 
 inline const Cell &TileLayer::cellAt(QPoint point) const