fixed upgrade mesh-renderer textures (cocos#6311)

Fix setting Rich Text image string invalid for 2d-tasks/issues/2566 (c…

…ocos#6274)

* Fix setting Rich Text image string invalid for 2d-tasks/issues/2566

* refine code

* refine code

Merge pull request cocos#6269 from caryliu1999/v2.3.1-release-fix-burst

Fix a problem of Burst for 3d particle.

Merge pull request cocos#6213 from 2youyou2/v2.3.1-math-api

fixed math api

fix tiledmap image layer load throw error (cocos#6171)

* change code style

* change code style

disable retina in editor for 2d-tasks/issues/2449 (cocos#6124)

Merge pull request cocos#6088 from 2youyou2/v2.3.0-delete-effect

fixed error when effect was deleted