VK_EXT_image_robustness requires returning zero on out-of-bounds image
accesses. OpImageFetch was previously merely clamping the coordinates to
be in-bounds.

This change reuses some of the functionality for
VK_SAMPLER_ADDRESS_MODE_CLAMP_TO_BORDER to perform texel replacement.

Bug: b/159329067
Tests: dEQP-VK.robustness.image_robustness.*
Change-Id: I8c00b8de2793b0b7028230cb180d308a4b9b60ec
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/47095
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>