The mouse cursor is hard to see on transparent backgrounds about slate

[mitchcurtis]

Something like this could work, but I think that fixing #47 might be a better solution for this.

diff --git a/app/imagecanvas.cpp b/app/imagecanvas.cpp
index 1cfb715..e8de142 100644
--- a/app/imagecanvas.cpp
+++ b/app/imagecanvas.cpp
@@ -132,6 +132,8 @@ ImageCanvas::ImageCanvas() :
     mCheckerPixmap = QPixmap(":/images/checker.png");
     if (mCheckerPixmap.isNull()) {
         qWarning() << "Failed to load checker pixmap";
+    } else {
+        mCheckerImage = mCheckerPixmap.toImage();
     }
 
     installEventFilter(this);
@@ -504,7 +506,13 @@ QColor ImageCanvas::cursorPixelColour() const
 
 QColor ImageCanvas::invertedCursorPixelColour() const
 {
-    return invertedColour(mCursorPixelColour);
+    if (mCursorPixelColour.alpha() > 0)
+        return invertedColour(mCursorPixelColour);
+
+    const int checkerX = mCursorSceneX / mCheckerImage.width();
+    const int checkerY = mCursorSceneY / mCheckerImage.height();
+    qDebug() << checkerX << checkerY << mCheckerImage.pixelColor(checkerX, checkerY);
+    return invertedColour(mCheckerImage.pixelColor(checkerX, checkerY));
 }
 
 QColor ImageCanvas::invertedColour(const QColor &colour)
diff --git a/app/imagecanvas.h b/app/imagecanvas.h
index fbc17da..5798677 100644
--- a/app/imagecanvas.h
+++ b/app/imagecanvas.h
@@ -398,6 +398,7 @@ protected:
     QColor mGridColour;
     QColor mSplitColour;
     QPixmap mCheckerPixmap;
+    QImage mCheckerImage;
 
     bool mSplitScreen;
     Splitter mSplitter;