- Make the various virtual methods of GimpPaintCore use a list of
  drawables as argument instead of a single drawable.
- gimp_brush_core_eval_transform_dynamics() can work with an image as
  argument rather than a drawable as it doesn't actually depends on
  specific drawable data.
- New function gimp_paint_tool_enable_multi_paint() to be used in init()
  method of paint tools to announce that this tool can work with
  multiple layers selected.
- Use gimp_paint_tool_enable_multi_paint() in the GimpSourceTool base
  class only for now.

This is a first step for multi-layer drawing, but we don't want it to be
possible in just any random cases, which is why I add a special function
to advertize this capability. We will use it for special-casing the
clone (as well as heal and perspective tools most likely) tool to work
on several layers at once. At this step, it is still very bugged and not
really working properly. In particular, since we don't process the
drawable offset early anymore (because it makes no sense when we pass a
list of drawables with different offsets), I suspect that all the
offset-related code will be very broken.