Fixed transparent blue screen (NME without OpenGL)

For some unrelated reason, I don't have OpenGL on my Windows 8.1. Therefore, OpenFL/NME will fallback to a without-OpenGL mode, and use SDL for this. **To test this or reproduce the bug, you have to disable OpenGL somehow**.

In this case however, my OpenFL window was blue, and the drawings not erased (for instance, antialiased text would get pixellised because semi-transparent antialias sums up until it becomes opaque).

I investigated, and it was in fact that somehow the alpha channel of the background color was interpreted as the blue channel, and the other colors were also swapped.

Then I found that the SDL surface was created with custom color masks, and changing them fixed the problem **on my computer**. I didn't have the occasion to test this on other targets yet.