plugins/screenshot: Invert render transforms in the correct order

The render transform is applied first, after that, the texture is
flipped implicitly by opengl.

So in order to undo these transforms, we need to flip the image
vertically first, then undo the render target transform.

src/plugins/screenshot/screenshot.cpp

@@ -76,10 +76,10 @@ static void convertFromGLImage(QImage &img, int w, int h, const OutputTransform
     }
 
     QMatrix4x4 matrix;
-    // OpenGL textures are flipped vs QImage
-    matrix.scale(1, -1);
     // apply render target transformation
     matrix *= renderTargetTransformation.inverted().toMatrix();
+    // OpenGL textures are flipped vs QImage
+    matrix.scale(1, -1);
     img = img.transformed(matrix.toTransform());
 }