Add GLTexture class. This is a convenient wrapper around managing an OpenGL texture, as well as loading an image into it. Includes support for loading from QImage, QPixmap, and a file. Defaults to a GL_TEXTURE_2D target, and will scale image if necessary.

Also add SceneOpenGL::Texture class, based on GLTexture. Optimised for SceneOpenGL::Window, this adds support for loading from an X Pixmap, as well as taking advantage of texture_from_pixmap/shm when available. Automatically detects what texture target should be used, so be sure to enableUnnormalizedTexCoords() before painting.
Make SceneOpenGL::Window, BoxSwitchEffect, and ExplosionEffect use the new classes.