Fixed a bug where calling Texture::getMaximumSize() before any GlResource is created would break context management.
Made a few functions in Texture and Shader a bit more thread-safe.
Added notes to documentation and adjusted OpenGL example as a workaround for the side effects of making static functions that require a context work. If the user wants to make use of both sfml-graphics and OpenGL, they should make sure sf::Texture::getMaximumSize() and/or sf::Shader::isAvailable() are called at least once before setting their final context active as those functions will cause a context switch the first time they are called.
Merge branch 'bugfix/texture_max_size'