An Implementation for Generic Blend Modes

[zsbzsb]

Once we have a clean branch I can also write the SFML.NET side of it. 

[Nexus]

There is now a new branch blendmodes based on thomas9459's commit.

I also adapted the test example to the newest API and created an updated Gist.

[Nexus]

We plan to merge the blendmodes branch to master for the SFML 2.2 release.

Has anybody worked with the new blend modes yet? Feel encouraged to use it and give feedback, now there's still the possibility to adapt smaller things

[Laurent]

I've reviewed the branch, and pushed my modifications. Nothing exciting: I added an overloaded constructor as discussed before, and made a few minor modifications to the comments. I think it's ok for merging, unless other members of the team want to have a look before.

[binary1248]

I checked the complete diff. Looks fine to me .