fix issue flutter/flutter#105392

In the current implementation, the snapshot_delegate of all lightweight engines is the same object - the rasterizer of the first engine. And when the FlutterViewController with first engine is not visible, then the surface_ of rasterizer will be set to nullptr. At this time, if other engines call Rasterizer::DoMakeRasterSnapshot, the issue will occur.

proposed solution

Lightweight engines no longer share snapshot_delegate, each engine's snapshot_delegate is its own rasterizer.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the [CLA].
• All existing and new tests are passing.