Depth bias support for WebGPU and refactor to add it to the DepthState

[mvaligursky]

Fixes #5688

• depth bias is supported on WebGPU (both Material and Shadow map)
• bias values are now part of the DepthState to avoid some conditions for each draw call - lights now stores the depth state used by it's shadow renderer, and that contains the shadow bias
• removed private API no longer needed from GraphicsDevice: setDepthBias and setDepthBiasValues
• updated MeshDecals example to not offset the plane to avoid z-fighting with decals, but to use bias on the decal material