SSR samples depth with linear sampler, which is poorly supported

## Bevy version and features

main

## What went wrong

SSR uses linear sampler for depth texture on native platforms:

https://github.com/bevyengine/bevy/blob/90b41543e2d0eadd94b52d99f184aa319f1de2e8/crates/bevy_pbr/src/ssr/mod.rs#L581-L582

https://github.com/bevyengine/bevy/blob/90b41543e2d0eadd94b52d99f184aa319f1de2e8/crates/bevy_pbr/src/ssr/raymarch.wgsl#L35-L37

WebGPU disallows it because it's poorly supported on native platforms: https://github.com/gpuweb/gpuweb/discussions/5151#discussioncomment-12795978

This is also a wgpu bug that it's not validated https://github.com/gfx-rs/wgpu/issues/8513

So we should manually sample depth to emulate linear filtering on native platforms as well as webgpu.