wayland: Use correct bounds size when computing the source box

The buffer transform specifies a transform from the buffer coordinate
space to the surface coordinate space.

The inverse buffer transform specifies a transform from the surface
coordinate space to the buffer coordinate space.

OutputTransform::map(QRect, QSizeF) expects both arguments to be in the
same coordinate space.

In case of SurfaceInterfacePrivate::computeSourceBox(), both should be
scaled surface coordinates so bufferTransform.inverted() maps the source
rect to the proper buffer coordinate space.

src/core/output.cpp

@@ -142,6 +142,19 @@ QRectF OutputTransform::map(const QRectF &rect, const QSizeF &bounds) const
     return dest;
 }
 
+QSizeF OutputTransform::map(const QSizeF &size) const
+{
+    switch (m_kind) {
+    case Kind::Normal:
+    case Kind::Rotated180:
+    case Kind::Flipped:
+    case Kind::Flipped180:
+        return size;
+    default:
+        return size.transposed();
+    }
+}
+
 Output::Output(QObject *parent)
     : QObject(parent)
 {

src/core/output.h

@@ -75,6 +75,11 @@ public:
      */
     OutputTransform inverted() const;
 
+    /**
+     * Applies the output transform to the given @a size.
+     */
+    QSizeF map(const QSizeF &size) const;
+
     /**
      * Applies the output transform to the given @a rect within a buffer with dimensions @a bounds.
      */

src/wayland/surface_interface.cpp

@@ -486,12 +486,13 @@ QRectF SurfaceInterfacePrivate::computeBufferSourceBox() const
         return QRectF(0, 0, bufferSize.width(), bufferSize.height());
     }
 
+    const QSizeF bounds = current.bufferTransform.map(bufferSize);
     const QRectF box(current.viewport.sourceGeometry.x() * current.bufferScale,
                      current.viewport.sourceGeometry.y() * current.bufferScale,
                      current.viewport.sourceGeometry.width() * current.bufferScale,
                      current.viewport.sourceGeometry.height() * current.bufferScale);
 
-    return current.bufferTransform.inverted().map(box, bufferSize);
+    return current.bufferTransform.inverted().map(box, bounds);
 }
 
 void SurfaceState::mergeInto(SurfaceState *target)