Don't reset stamp brush state when pressing Alt

This way, Alt can be used to toggle the scrolling direction without
interrupting a Paint or Capture operation.

Also, while drawing lines or circles, pressing Alt no longer resets
the state when a start or mid point was already set.

See https://twitter.com/Lanbobyonson/status/978015338582376448

NEWS.md

@@ -7,6 +7,7 @@
 * macOS: Fixed eye/lock icon display in Layers view
 * Re-enabled Space for toggling layer visibility
 * Migrate properties set on tile collision layer to the tile (#1912)
+* Don't reset stamp brush state when pressing Alt
 * Automapping: Apply rules to selected area when there is one
 * Windows and Linux: Updated builds to Qt 5.10.1
 * Linux: Indicate Tiled can open multiple files at once in desktop file

src/tiled/stampbrush.cpp

@@ -170,25 +170,30 @@ void StampBrush::modifiersChanged(Qt::KeyboardModifiers modifiers)
     if (mStamp.isEmpty() && !mIsWangFill)
         return;
 
+    BrushBehavior brushBehavior = mBrushBehavior;
+
     if (modifiers & Qt::ShiftModifier) {
         if (modifiers & Qt::ControlModifier) {
-            if (mBrushBehavior == LineStartSet) {
-                mBrushBehavior = CircleMidSet;
-            } else {
-                mBrushBehavior = Circle;
+            if (brushBehavior == LineStartSet) {
+                brushBehavior = CircleMidSet;
+            } else if (brushBehavior != CircleMidSet) {
+                brushBehavior = Circle;
             }
         } else {
-            if (mBrushBehavior == CircleMidSet) {
-                mBrushBehavior = LineStartSet;
-            } else {
-                mBrushBehavior = Line;
+            if (brushBehavior == CircleMidSet) {
+                brushBehavior = LineStartSet;
+            } else if (brushBehavior != LineStartSet) {
+                brushBehavior = Line;
             }
         }
-    } else {
-        mBrushBehavior = Free;
+    } else if (brushBehavior != Paint && brushBehavior != Capture) {
+        brushBehavior = Free;
     }
 
-    updatePreview();
+    if (mBrushBehavior != brushBehavior) {
+        mBrushBehavior = brushBehavior;
+        updatePreview();
+    }
 }
 
 void StampBrush::languageChanged()