Fixed Texture::getMaximumSize() breaking context management

Fixed a bug where calling Texture::getMaximumSize() before any GlResource is created would break context management.
`static` is not thread-safe. This might be okay given the semantics of `sf::Texture`, but I want to make sure that you're aware that calling `Texture::getMaximumSize()` from multiple threads is dangerous. In other places, there seem to be mutexes at least...