Fix custom pass scaling issues in after post process

[alelievr]

Purpose of this PR

Fix an issue regarding the scaling of texture read from the after-post-process injection point.

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Testing status

Tested that we can read the camera main color in a custom pass with the injection point after post-process and tested several dynamic resolution settings and upsampling algorithms and DLSS

https://unity-ci.cds.internal.unity3d.com/job/11824094/dependency-graph

[github-actions]

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[TomasKiniulis]

Looks good! Checked that no scaling issues occur when camera is looking directly into the planar probe when fullscreen shader writes color from Custom Color buffer. Tested Custom Pass on all injection points.

[sebastienlagarde]

replicated on private