Fix a bug with slice in webgl backend

[dsmilkov]

Fixing a bug in the WebGL backend where we can release a texture upon reading it, regardless of whether it was sliced or not. In js code:

const a = tf.tensor([1, 2]);
const b = a.slice([0]);
await a.read(); // Would release the texture to the manager.
await b.read(); // Would still work by chance since it holds the pointer to the `WebGLTexture` and the texture manager hasn't reused it yet.

The fix removes all direct calls to this.releaseTexture() and wraps releaseTexture() in a smarter this.releaseGPUData() which will check if there are sliced tensors depending on it before releasing.

Added a unit test that reproduced the issue and is now passing.

BUG

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This change is 

[nsthorat]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! 1 of 1 approvals obtained (waiting on @nsthorat)