Fixed Texture::getMaximumSize() breaking context management

Fixed a bug where calling Texture::getMaximumSize() before any GlResource is created would break context management.
This is only an issue because we are still in C++03 :wink:. I agree that there might theoretically be problems, but practically? I haven't noticed any myself. The only way to get the variables to be written to (which is the only problem) simultaneously is to call getMaximumSize() for the "first" time from 2 different threads.

I don't know if protecting against this special case is worth it considering it will probably be removed once SFML moves to C++11.