Stop using illegal negative values for wgpu::SamplerDescriptor::lod_min_clamp.

[James2022-rgb]

Negative values like -100.0 are illegal for GPUSamplerDescriptor.lodMinClamp.

More specifically, WebGPU valid usage states that lodMinClamp must not be negative, and lodMaxClamp must be >= lodMinClamp:

descriptor.lodMinClamp ≥ 0.
descriptor.lodMaxClamp ≥ descriptor.lodMinClamp.

Indeed, Chrome's WebGPU implementation complains:

LOD clamp bounds [-100.000000, 100.000000] contain contain a negative number.
 - While validating [SamplerDescriptor]
 - While calling [Device].CreateSampler([SamplerDescriptor]).

It dislikes it even more when we actually try to sample using this illegal sampler:

[Invalid Sampler] is invalid.
 - While validating entries[1] as a Sampler.
Expected entry layout: { binding: 1, visibility: ShaderStage::Fragment, sampler: { type: SamplerBindingType::Filtering } }
 - While validating [BindGroupDescriptor "material_bind_group"] against [BindGroupLayout "material_bind_group_layout"]
 - While calling [Device].CreateBindGroup([BindGroupDescriptor "material_bind_group"]).

[Invalid BindGroup "material_bind_group"] is invalid.
 - While encoding [RenderPassEncoder "Render Pass"].SetBindGroup(1, [BindGroup], 0, ...).

[Invalid CommandBuffer] is invalid.
    at ValidateObject (..\..\third_party\dawn\src\dawn\native\Device.cpp:651)
    at ValidateSubmit (..\..\third_party\dawn\src\dawn\native\Queue.cpp:393)

This PR changes all the illegal negative values to 0.0.

The value of 0.0 is the default value for ``wgpu::SamplerDescriptor::lod_min_clamp` anyway so technically it can be omitted,
but I think being explicit here helps for documentation purposes.

[James2022-rgb]

Note that this is a hard error now on wgpu master branch:
gfx-rs/wgpu#3353

Not sure when it will be on crates.io.

[sotrh]

Awesome! Thanks!