Change gradient generator for pipeline image tests

Change gradient generator for pipeline image tests

Pipeline image tests were initializing images with a
gradient generator which used a single image dimension
per channel (i.e. red channel depending only on x).
This was problematic when testing formats that were not
using all the channels. For example layer tests were
still passing for R16 formats even when sampling was
forced to always use layer 0 because the layer index
was encoded in a missing blue channel.

There was another gradient generator available that
encoded all coordinates into each channel solving
this problem.

VK-GL-CTS Issue: 2525

Affects:

dEQP-VK.pipeline.image.*

Components: Vulkan
Change-Id: Ie5ff0ddc5c373577952a4240bd10e10b9f25507b