Add the ability to specify a filter type on BaseTexture

This should allow users to specify `PIXI.Texture.FILTER.NEAREST` as the filter type for a base texture to get nearest filtering.

The default is to continue doing what has always been done (linear webgl filtering and bicubic canvas filtering). You can also manually specify `PIXI.Texture.FILTER.LINEAR` as the filter type to get the default mode of operation.