WebGPURenderer: cache renderPassDesciptors and associated views.

[aardgoose]

For the default null renderTarget cache descriptor, depth texture view and msaa texture view where appropriate.
For non null targets, use the renderTarget as a key for caching renderPassDescriptors to reduce per pass object creation .

The default descriptor members could be reused in the clear() method.

[sunag]

Some examples is warn the message bellow like webgpu_compute_particles_rain, webgpu_compute_particles_snow:

Both stencilLoadOp (LoadOp::Clear) and stencilStoreOp (StoreOp::Store) must not be set if the attachment ([TextureView]) has no stencil aspect or stencilReadOnly (0) is true.
 - While validating depthStencilAttachment.
 - While encoding [CommandEncoder "renderContext_2"].BeginRenderPass([RenderPassDescriptor]).

[aardgoose]

I'll have a look

[aardgoose]

Renderer now sets correct depth and stencil values for renderContext objects, and depth / depth+stencil objects created as required.