WebGLRenderer: Move invocation of Scene.onAfterRender().

[Mugen87]

Related issue: Fixed #17761.

Description

The reason for this change is explained in #17710.

scene.onAfterRender must be called after multiview or multisampled rendertargets do their blitFramebuffer for post proc to have valid data in the renderTarget.

The original PR was hard to test because it contained too many changes at once. #18626 implemented one part of it. This PR is about onAfterRender(). The changes related to WebGLMutliview are not relevant anymore. And the issue in WebGLTextures should be solved via #18904.

[mrdoob]

Thanks!