app: add GimpBrushCache which stores brush stuff based on transform parameters
such as masks and outlines. The cache is currently very stupid and only cacheds the last transformed object. Add caches to GimpBrush for its mask, its pixmap and its boundary, and remove the same caches and a ton of members from GimpBrushCore. This involves adding lots of const qualifiers because GimpBrush returns const pointers now for trasnformed stuff.
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