Add flag to avoid creating backround layers

Now callers can force loading a file without creating any background
layer in the doc. This is because for drag and drop operations we don't
want to generate background layers

src/app/cmd/drop_on_timeline.cpp

@@ -81,13 +81,18 @@ void DropOnTimeline::onExecute()
   LayerGroup* group = nullptr;
 
   int flags =
-    FILE_LOAD_DATA_FILE |
+    FILE_LOAD_DATA_FILE | FILE_LOAD_AVOID_BACKGROUND_LAYER |
     FILE_LOAD_CREATE_PALETTE | FILE_LOAD_SEQUENCE_YES;
 
   int fopCount = 0;
   while(!m_paths.empty()) {
     std::unique_ptr<FileOp> fop(
       FileOp::createLoadDocumentOperation(context, m_paths.front(), flags));
+
+    // Do nothing (the user cancelled or something like that)
+    if (!fop)
+      return;
+
     fopCount++;
 
     base::paths fopFilenames;
@@ -99,10 +104,6 @@ void DropOnTimeline::onExecute()
         m_paths.erase(it);
     }
 
-    // Do nothing (the user cancelled or something like that)
-    if (!fop)
-      return;
-
     if (fop->hasError()) {
       console.printf(fop->error().c_str());
     }

src/app/file/file.cpp

@@ -533,6 +533,11 @@ FileOp* FileOp::createLoadDocumentOperation(Context* context,
       fop->m_dataFilename = dataFilename;
   }
 
+  // Avoid creating a background layer?
+  if (flags & FILE_LOAD_AVOID_BACKGROUND_LAYER) {
+    fop->m_avoidBackgroundLayer = true;
+  }
+
 done:;
   return fop.release();
 }
@@ -939,8 +944,9 @@ void FileOp::operate(IFileOpProgress* progress)
 
       // Final setup
       if (m_document) {
-        // Configure the layer as the 'Background'
-        if (!m_seq.has_alpha)
+        // Configure the layer as the 'Background'. Only if background layers
+        // are welcome.
+        if (!m_seq.has_alpha && !m_avoidBackgroundLayer)
           m_seq.layer->configureAsBackground();
 
         // Set the final canvas size (as the bigger loaded
@@ -1522,6 +1528,7 @@ FileOp::FileOp(FileOpType type,
   , m_oneframe(false)
   , m_createPaletteFromRgba(false)
   , m_ignoreEmpty(false)
+  , m_avoidBackgroundLayer(false)
   , m_embeddedColorProfile(false)
   , m_embeddedGridBounds(false)
 {

src/app/file/file.h

@@ -27,13 +27,14 @@
 #include <string>
 
 // Flags for FileOp::createLoadDocumentOperation()
-#define FILE_LOAD_SEQUENCE_NONE         0x00000001
-#define FILE_LOAD_SEQUENCE_ASK          0x00000002
-#define FILE_LOAD_SEQUENCE_ASK_CHECKBOX 0x00000004
-#define FILE_LOAD_SEQUENCE_YES          0x00000008
-#define FILE_LOAD_ONE_FRAME             0x00000010
-#define FILE_LOAD_DATA_FILE             0x00000020
-#define FILE_LOAD_CREATE_PALETTE        0x00000040
+#define FILE_LOAD_SEQUENCE_NONE          0x00000001
+#define FILE_LOAD_SEQUENCE_ASK           0x00000002
+#define FILE_LOAD_SEQUENCE_ASK_CHECKBOX  0x00000004
+#define FILE_LOAD_SEQUENCE_YES           0x00000008
+#define FILE_LOAD_ONE_FRAME              0x00000010
+#define FILE_LOAD_DATA_FILE              0x00000020
+#define FILE_LOAD_CREATE_PALETTE         0x00000040
+#define FILE_LOAD_AVOID_BACKGROUND_LAYER 0x00000080
 
 namespace doc {
   class Tag;
@@ -285,6 +286,8 @@ namespace app {
     bool newBlend() const { return m_config.newBlend; }
     const FileOpConfig& config() const { return m_config; }
 
+    bool avoidBackgroundLayer() const { return m_avoidBackgroundLayer; }
+
   private:
     FileOp();                   // Undefined
     FileOp(FileOpType type,
@@ -314,6 +317,7 @@ namespace app {
                                 // GIF/FLI/ASE).
     bool m_createPaletteFromRgba;
     bool m_ignoreEmpty;
+    bool m_avoidBackgroundLayer;
 
     // True if the file contained a color profile when it was loaded.
     bool m_embeddedColorProfile;

src/app/file/fli_format.cpp

@@ -95,7 +95,8 @@ bool FliFormat::onLoad(FileOp* fop)
   Sprite* sprite = new Sprite(ImageSpec(ColorMode::INDEXED, w, h), 256);
   LayerImage* layer = new LayerImage(sprite);
   sprite->root()->addLayer(layer);
-  layer->configureAsBackground();
+  if (!fop->avoidBackgroundLayer())
+    layer->configureAsBackground();
 
   // Set frames and speed
   sprite->setTotalFrames(frame_t(header.frames));

src/app/file/gif_format.cpp

@@ -288,7 +288,7 @@ public:
           break;
       }
 
-      if (m_layer && m_opaque)
+      if (m_layer && m_opaque && !m_fop->avoidBackgroundLayer())
         m_layer->configureAsBackground();
 
       // sRGB is the default color space for GIF files

src/app/file/webp_format.cpp

@@ -211,7 +211,7 @@ bool WebPFormat::onLoad(FileOp* fop)
   }
   WebPAnimDecoderReset(dec);
 
-  if (!has_alpha)
+  if (!has_alpha && !fop->avoidBackgroundLayer())
     layer->configureAsBackground();
 
   WebPAnimDecoderDelete(dec);