Test image load/store with packed formats

Test image load/store with packed formats

This CL adds image load/store tests for the packed formats
requiring the StorageImageExtendedFormats SPIR-V capability.
The thresholds in the result verification are now based
on the channel class and bit depths of the components.

The shader generation is modified to store also negative
values on signed formats.

New tests:

dEQP-VK.image.store.*pack32*
dEQP-VK.image.load_store.*pack32*
dEQP-VK.image.format_reinterpret.*pack32*

Affects:

dEQP-VK.image.store.*
dEQP-VK.image.load_store.*
dEQP-VK.image.format_reinterpret.*
dEQP-VK.image.qualifiers.restrict.*

Components: Framework, Vulkan

VK-GL-CTS issue: 1814

Change-Id: Ia2d061dbfd9c19ade8057398dd78692b5a28c029