… and gimp_brush_get_mask().

gimp_brush_get_pixels() was a bit crappy, returning raw data with only
dimensions and bpp to go with (no color model/space, no bit depth…). So the
assumption is that we work with 8-bit per channel data, possibly with alpha
depending of number of channels as deduced from bpp, and very likely in sRGB
color space. It might be globally ok with many of the brush formats (and
historical brushes) but won't fare well as we improve brush capabilities.

- gimp_brush_get_pixels() is in fact made private.
- The 2 new functions are using this old PDB call _gimp_brush_get_pixels() to
  construct buffers. This has some limitations, in particular that it returns
  only 8-bit per channel sRGB data, but at least the signature won't change when
  we will improve things in the future (so if some day, we pass fancy brushes in
  high-bit depth, the method will stay the same).
- This new implementation also allows scaling down the brush (keeping aspect
  ratio) which is useful when you need to fit a brush preview into a drawing
  widget.
- Current implementation stores the buffers at native size in the libgimp's
  GimpBrush object, hence save re-querying the core every time you need an
  update. This can be improved as current implementation also means that you
  don't get updates if the brush changed. This should handle most common use
  cases for now, though.
- Also with this change, I move GimpBrush class implementation into its own
  dedicated file.