Make images contribute to Picture allocation size, more aggressively release references when dispose is called (#18706)

SkImage references get held by both our Image and Picture objects. The GC has no idea about this, and so sometimes pictures look very small when they're actually holding a reference to a large chunk of memory. This patch helps make sure that the GC can more adequately catch the real size impact of Picture objects, and combined with an upstream patch in Dart allows for much more aggressive collection of unused image related memory.