Texture & buffer call now destroy on removal from pool

[Wumpf]

From WebGPU Spec:

An application that no longer requires a GPUTexture can choose to lose access to it before garbage collection by calling destroy().
Note: This allows the user agent to reclaim the GPU memory associated with the GPUTexture once all previously submitted operations using it are complete.

(https://www.w3.org/TR/webgpu/#dom-gputexture-destroy)
(similiar for Buffer https://www.w3.org/TR/webgpu/#buffer-destruction)

Fixes #592

Checklist

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