In GimpPaintbrush, factor out the code responsible for determining
the current dab's paint parameters and content to a new
GimpPaintbrush::get_paint_params() virtual function.

Derive GimpEraser from GimpPaintbrush, instead of directly from
GimpBrushCore, implementing get_paint_params() appropriately.
This allows GimpEraser to reuse the paint-buffer content across
dabs, improving performance.