Change readPixels to leave out-of-bounds pixels untouched rather than zeroing them

In this discussion:
https://www.khronos.org/webgl/public-mailing-list/archives/1511/msg00002.php

it was decided to change readPixels' behavior to leave pixels untouched that are outside the range of the bound framebuffer, rather than zeroing them. The WebGL 1.0 and 2.0 specs need to be updated, as do the conformance tests.