Fix cross fade transition

[rh101]

Which branch your pull-request should merge into?

• 2.1: BugFixes and Improvements for Current LTS branch

Describe your changes

This is related to the issue described in #1663 with the cross-fade transition. The render textures used for the transition cannot use a shared render target. It's reproducible with the cpp-tests project (test menu 64: Transitions, test 8).

Issue ticket number and link

Checklist before requesting a review

For each PR

• Add Copyright if it missed:
  - "Copyright (c) 2019-present Axmol Engine contributors (see AUTHORS.md)."

• I have performed a self-review of my code.

  Optional:

  • I have checked readme and add important infos to this PR.
  • I have added/adapted some tests too.

For core/new feature PR

• I have checked readme and add important infos to this PR.
• I have added thorough tests.

[aismann]

@rh101
Plz add the copyright line too:
"Copyright (c) 2019-present Axmol Engine contributors (see AUTHORS.md)."

Thanks.