actually draw drag icons in the mouse cursor during drags

Summary:
paint together the cursor image and the extra dnd image if available
on X11 with QDrag::setHotspot is possible to control their relative
position, which doesn't seem to have a wayland protocol correspondence
so their relative position are controlled just by the hotspot of the cursor itself

Test Plan:
folder graphics from dolphin is painted correctly
{F8086937}

Reviewers: #plasma, #kwin, davidedmundson

Reviewed By: #plasma, #kwin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D27174

pointer_input.cpp

@@ -51,6 +51,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 #include <QHoverEvent>
 #include <QWindow>
+#include <QPainter>
 // Wayland
 #include <wayland-cursor.h>
 
@@ -1221,6 +1222,7 @@ void CursorImage::updateDragCursor()
         if (auto dragIcon = ddi->icon()) {
             if (auto buffer = dragIcon->buffer()) {
                 additionalIcon = buffer->data().copy();
+                additionalIcon.setOffset(dragIcon->offset());
             }
         }
     }
@@ -1253,7 +1255,32 @@ void CursorImage::updateDragCursor()
         return;
     }
     m_drag.cursor.hotSpot = c->hotspot();
-    m_drag.cursor.image = buffer->data().copy();
+
+    if (additionalIcon.isNull()) {
+        m_drag.cursor.image = buffer->data().copy();
+    } else {
+        QRect cursorRect = buffer->data().rect();
+        QRect iconRect = additionalIcon.rect();
+
+        if (-m_drag.cursor.hotSpot.x() < additionalIcon.offset().x()) {
+            iconRect.moveLeft(m_drag.cursor.hotSpot.x() - additionalIcon.offset().x());
+        } else {
+            cursorRect.moveLeft(-additionalIcon.offset().x() - m_drag.cursor.hotSpot.x());
+        }
+        if (-m_drag.cursor.hotSpot.y() < additionalIcon.offset().y()) {
+            iconRect.moveTop(m_drag.cursor.hotSpot.y() - additionalIcon.offset().y());
+        } else {
+            cursorRect.moveTop(-additionalIcon.offset().y() - m_drag.cursor.hotSpot.y());
+        }
+
+        m_drag.cursor.image = QImage(cursorRect.united(iconRect).size(), QImage::Format_ARGB32_Premultiplied);
+        m_drag.cursor.image.fill(Qt::transparent);
+        QPainter p(&m_drag.cursor.image);
+        p.drawImage(iconRect, additionalIcon);
+        p.drawImage(cursorRect, buffer->data());
+        p.end();
+    }
+
     if (needsEmit) {
         emit changed();
     }