Add proper support for sRGB

[eXpl0it3r]

Due to the way the CSFML API was implemented for creating/loading textures, there was no way to create textures in sRGB mode and the exposed setSrgb method was essentially useless.

With SFML/CSFML#177 this was fixed, which then however leaves a gap in SFML.NET. The property for sRGB needs to be adapted and the new creation functions need to be called from SFML.NET.