High CPU usage when using cards from multiple vendors.

[MaxDZ8]

The miner thread currently pushes work as hard as possible until all hashing tasks signal to be on a "wait for data transfer" stage.
Those signals are pooled in a single set used to sleep.
Unfortunately, this is not a valid operation when wait events come from different contexts. This causes wait to fail which in turn causes core usage go 100%.
A context is internal to each platform so this issue cannot be easily solved.
Possible solutions include:

• Have a set of parallel threads waking up miner thread on first completion: relatively efficient but a lot of work for little benefit;
• Just sleep for 1ms
• clFinish

Latter 2 would need to be better investigated.
In the end, this does not seem to be something I want to support, much less implement soon.