Merge redundant view-binding samplers to fix WebGPU sampler limit overflow

[issam3105]

Objective

Partially addresses #24711 — reduces the number of sampler bindings in @group(0) of the mesh view bind group.

WebGPU impose tight limits on the number of samplers per shader stage (often 16). Before this PR, every feature that needed texture sampling allocated its own dedicated Sampler object and binding slot, leaving little headroom for features such as additional glTF extensions (e.g. KHR_materials_transmission, KHR_materials_specular, KHR_materials_anisotropy...) that would each require their own sampler.

Solution

Audit every sampler in @group(0) and merge those that share the same descriptor:

• Introduce a single LinearSampler resource (always bound at slot 25) that replaces all per-feature Linear samplers.
• Merge the two Comparison shadow samplers (point + directional were identical) into one shadow_comparison_sampler.
• Remove the redundant Sampler fields from ShadowSamplers, AtmosphereSampler, and ViewTransmissionTexture.

Testing

• Tested an HTML5 app on WebGPU: before this PR, enabling pbr_transmission_textures triggered:

  Caught rendering error: The number of samplers (19) in the Fragment stage exceeds the maximum per-stage limit (16).
  

  After this PR, the same app runs without error and features such as pbr_specular_textures + pbr_transmission_textures can be enabled simultaneously.
• Tested on Windows (Vulkan).

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Showcase

Mapping from old to shared samplers

Binding	Before	After
3	point_shadow_textures_comparison_sampler	shadow_textures_comparison_sampler
4	point_shadow_textures_linear_sampler (pcss)	linear_sampler
6	directional_shadow_textures_comparison_sampler	shadow_textures_comparison_sampler
7	directional_shadow_textures_linear_sampler (pcss)	linear_sampler
19	dt_lut_sampler (tonemap)	dt_lut_sampler (tonemap, unchanged)
25	view_transmission_sampler (conditional)	linear_sampler
32	atmosphere_transmittance_sampler (conditional)	linear_sampler
36	area_light_luts_sampler (conditional)	linear_sampler
38	dfg_lut_sampler (conditional)	linear_sampler

9 → 3 sampler bindings (with all features active).

GPU Sampler objects created at startup

Before	After
point_light_comparison_sampler	shadow_comparison_sampler
directional_light_comparison_sampler	shadow_comparison_sampler
point_light_linear_sampler (pcss)	LinearSampler
directional_light_linear_sampler (pcss)	LinearSampler
AtmosphereSampler	LinearSampler
ViewTransmissionTexture::sampler	LinearSampler

6 → 2 GPU sampler objects.

[issam3105]

Should i compact the binding indices to fill the gaps left by the removed sampler ?