Improve tests for minSampleShading

Improve tests for minSampleShading

Existing tests had chicken & egg problem: they were just comparing resolved
images rendered to with minSampleShading > 0 to resolved images rendered to with
minSampleShading == 1. This means that any tests could pass in case of
consistent bug in sample shading.

This change reworks the test do proper validation of number of unique
generated samples:
 - Renderpass is extended with passes for copying per-sample data into
 single sampled images

 - Single sampled images are than cross-checked on CPU: for each pixel,
 the number of covered unique samples accross all images must be not less
 than amoung expected from given minSampleShading value.

Affects:
dEQP-VK.pipeline.multisample.min_sample_shading.*

Components: Vulkan

VK-GL-CTS issue: 579

Change-Id: I24cbf471a499de5db39b028e598a13899114a3e4