Expose BitmapFilterProgram publically to allow external extend

Hi  Bernhard,

Here's is a little pull request that change put the class _BitmapFilterProgram into BitmapFilterProgram.
This let users extend the class be able to create their own WebGL filter out of the StabeXL library.

In my case, I would use this to create a WebGL filter that fit exactly my need, but it is too specific to be useful inside StageXL itself.