GREASED_LINE_USE_OFFSETS - WebGPU fix for Safari/Firefox

[RolandCsibrei]

See #16045

With offsets:
Simple shader material: https://playground.babylonjs.com/?snapshot=refs/pull/16053/merge#H1LRZ3#717
Material plugin: https://playground.babylonjs.com/?snapshot=refs/pull/16053/merge#H1LRZ3#122

Without offsets:
Simple shader material: https://playground.babylonjs.com/?snapshot=refs/pull/16053/merge#406E3H#29
Material plugin: https://playground.babylonjs.com/?snapshot=refs/pull/16053/merge#406E3H#23

Tested on MacOS Chrome and Safari, WebGL and WebGPU.

[bjsplat]

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.

[bjsplat]

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.

[RolandCsibrei]

@Popov72 The shaders can be simplified even more. I'll do it tomorrow.

[bjsplat]

Snapshot stored with reference name:
refs/pull/16053/merge

Test environment:
https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16053/merge/index.html

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

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16053/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

https://playground.babylonjs.com/?snapshot=refs/pull/16053/merge
https://sandbox.babylonjs.com/?snapshot=refs/pull/16053/merge
https://gui.babylonjs.com/?snapshot=refs/pull/16053/merge
https://nme.babylonjs.com/?snapshot=refs/pull/16053/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/16053/merge#BCU1XR#0

[bjsplat]

WebGL2 visualization test reporter:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16053/merge/testResults/webgl2playwright/index.html

[bjsplat]

Visualization tests for WebGPU (Experimental)

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/16053/merge/testResults/webgpuplaywright/index.html

[RolandCsibrei]

@Popov72 The shaders can be simplified even more. I'll do it tomorrow.

I thought I could optimize the shaders by removing the var grlPositionOffset = vec3f(0.); part and perform adding the offset only when there is an grl_offsets attribute present but I have just read about the WGSL compiler's constant folding optimization technique so I'll rather prefer readibility of the shader and decided not to add several #ifdefs to the shaders. PR is ready to review.