Fix firefox warning caused by invalid max anisotropy setting.

From the terrain branch...

If the `maximumAnisotropy` property was not specified in a call to `Texture.setSampler`, Cesium would end up calling this with a value of undefined:

`gl.texParameteri(target, this._textureFilterAnisotropic.TEXTURE_MAX_ANISOTROPY_EXT, s.maximumAnisotropy);`

In Chrome, this is silently ignored.  In Firefox, it prints a warning to the error console.

My simple fix is to use a value of 1 if `s.maximumAnisotropy` is falsy.