Use oct-encoding for `NormalPrepass` texture

## What problem does this solve or what need does it fill?

Currently normal prepass texture is `Rgb10A2`, which is 10x3 bit for normal vec3 and 2 bit unused. The precision is much lower than oct-encoded `Rg16Snorm` though oct-encoding/decoding it in shaders has a overhead.

## What solution would you like?

Use oct-encoded `Rg16Snorm` for normal prepass texture.

## What alternative(s) have you considered?

Leave it as is.

## Additional context

See also #21926 which uses oct-encoding for vertex normal and tangent.

Start at the NormalPrepass texture implementation and its shader consumers, then compare the oct-encoding approach referenced in #21926. Replace the Rgb10A2 representation with oct-encoded Rg16Snorm and verify that normal prepass rendering and decoding preserve the expected normal precision.

Written by the indexing model from the issue text.