Fix window resolution.
Also fix a bug with freeing surfaces; UnloadSurface() would remove the element from the map which would break the iterator in the caller.
More bugfixes.
 * LoadTexture() now loads plain textures.
 * GL_BLEND is now reset in Texture::Draw().
 * Display::InitFadeTextures() now sets the texture width/height correctly.
 * MenuBackgroundTexture::Draw() now enables GL_TEXTURE_2D.
 * Various initialisation.