Provide Image::copy, which handles copying rectangles from a source to a destination, with thorough bounds checking.
Also fixes an indexing error in SpriteAtlas, where the top row of pixels in a wrapped image was copied from the wrong source row.
Otherwise, it can participate in unexpected conversions. Case in point: GlyphSet::insert was comparing the result of GlyphMetrics::operator bool() where it wanted to use operator==.
