Wrong alpha channel on exported frames from renderer

[MathiasPaulin]

When exporting a frame from the renderer, the alpha-channel is the one resulting from the rendering of objects. When the scene contains transparent objects, or when the exported frame is a wireframe rendering, this results in images that do not display as expected.
To be able to display the image on any background (e.g. when included in a pdf to prevent black background), keeping alpha value of 0 where no objects are drawn is mandatory but not for any other values.

The following image show this problem :

What is expected is the following :

To better see the problem, just open the above png pictures with gimp or any display program.

The PR #857 fixes this.