[WGSL] Emit code gen for clamping depth to the viewport

This only impacts when frag_depth / ([[depth(any)]] ) is used in the shader.

Something like this I think would work:

struct FragOutput {
...
float depthValue [[depth(any)]];
};

void fragmentShader(const constant unsigned* __DynamicOffsets [[buffer(4)]]) {
  FragOutput o;
...
  float minDepth = as_type<float>(__DynamicOffsets[maxDynamicOffsetIndex + 1]);
  float maxDepth = as_type<float>(__DynamicOffsets[maxDynamicOffsetIndex + 2]);
  o.depthValue = clamp(o.depthValue, minDepth, maxDepth);
}