Split image bounds check policy

[teoxoy]

Split image bounds check policy since we shouldn't need bounds checks for writes.
See gpuweb/gpuweb#3893 (comment).

[jimblandy]

Since all the backends ignore writes anyway, the only thing one can do with an image write policy is ask for Restrict, which seems like something nobody actually prefers.

Wouldn't it make more sense to just rename BoundsCheckPolicy::image to image_load, and remove bounds checking on ImageStore altogether?

[teoxoy]

I wanted to first see the guarantee landed in the spec. Put up: gpuweb/gpuweb#4194

[jimblandy]

Looks good to me. We can just drop image_store if that comes to pass.