Proposal for transparent images tracking

[Carnaux]

I'm having issues when the image used to create the markers has mostly transparent backgrounds, that's because the SURF algorithm searches for the contrast between black on white or white on black, and that creates two opposites, mostly, white background images don't work with black background markers.

My proposal:

Detecting if the image has a mostly transparent background and adding that info on the marker files

Add a condition on the marker detection/tracking for the transparency flag

If the flag exists, use a background extraction algorithm and then a color replacement to white or to black while keeping the design colors(Like a green screen).

Didn't been tested yet, just a proposal and reminder of the issue. @kalwalt @ThorstenBux