Improve shader cache hashing

Improve shader cache hashing

This change adds FNV-1 hash algorithm and a check whether a shader is
already in the cache before writing it there. In multi-threaded shader
compilation the cache might have written multiple copies of the same
shader into the cache, wasting disk space.

Affects:

None

Components: Vulkan

VK-GL-CTS issue: 1641

Change-Id: Ideaed34f49994f5da50b75066e611e60023eccb1