Introduces the sf::BlendMode class that supports separate blend functions and equations.
Replaces the old sf::BlendMode enum.
Further changes:
* Fixed merge conflicts resulting from GLExtensions in mobile branch
* GLExtensions: Added GLEXT_blend_equation_separate
* Added file BlendMode.cpp
* Replaced occurrences of sf::BlendMode value parameters with const-references
* Adapted rendering code in sf::RenderTarget