PrePass renderer: Fix engine current render pass id set too early

[Popov72]

See https://forum.babylonjs.com/t/issue-when-prepassrenderer-and-rendertargettexture-are-used-at-the-same-time/29317/7

Basically it's a revert of my changes from:

269132c#diff-5fec6a90573f71adda2b2d7c9364ac6da73fe8cfc6a641b129a051bf38d67cd2

I don't remember why I did this change at that time, I think it's wrong to set Engine.currentRenderPassId in PrePassRenderer._setRenderTarget as RenderTargetTexture._renderToTarget will set the right pass id before drawing the meshes anyway.

[EDIT] Ok, I updated the fix which is not simply a revert now: the engine.currentRenderPassId needs to be updated but only when setting the default RT.

[azure-pipelines]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[azure-pipelines]

Snapshot stored with reference name:
refs/pull/12988/merge

Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12988/merge/index.html

To test a playground add it to the URL, for example:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12988/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/12988/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/12988/merge
https://gui.babylonjs.com/?snapshot=refs/pull/12988/merge
https://nme.babylonjs.com/?snapshot=refs/pull/12988/merge

To test the snapshot in the playground with a playground ID add it after the snapshot query string:

https://playground.babylonjs.com/?snapshot=refs/pull/12988/merge#BCU1XR#0

[azure-pipelines]

Visualization tests for webgl2 have failed. If some tests failed because the snapshots do not match, the report can be found at

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/12988/merge/testResults/webgl2/index.html

If tests were successful afterwards, this report might not be available anymore.