Set default queue priorities to 0.5f

[SaschaWillems]

Description

As discussed on the last sample call, always setting queue priorities to 1.0f (max.) is not a good practice. This PR adjusts all places where queue priorities are set to use 0.5f instead. Won't make a difference anyway, as we rarely (if at all) use multiple queues from the same family.

Refs KhronosGroup/Vulkan-Tutorial#166

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[asuessenbach]

I've probably missed something, but why is a queuePriority of 1.0f not a good practice?
As we never have more than one queue from the same family, the actual value doesn't make any difference. Why not keeping it at 1.0f?

[SaschaWillems]

We discussed this on one of our recent calls and agreed to change/make it consistent across all samples (and the tutorial). This is similar to other computing areas where telling people to set the highest priority for a resource is usually considered bad practice. People will copy that and at some point might wonder why things don't work out.

[gary-sweet]

I might have gone with 0.5, but 0.0 shouldn't cause any problems.