bitmaptools.alphablend: add optional L8 mask= kwarg

Adds an optional mask= keyword argument to bitmaptools.alphablend()
that scales source2's per-pixel contribution by an L8 bitmap (one
byte per pixel, 0..255). When mask is None (default), behavior is
unchanged from the existing alphablend.

Mask bitmap must match the output bitmap's width and height and have
a bits_per_value of 8.