The `vtkm_unit_tests` function in the CMake build now allows you to specify
which files need to be compiled with a device compiler using the
`DEVICE_SOURCES` argument. Previously, the only way to specify that unit
tests needed to be compiled with a device compiler was to use the
`ALL_BACKENDS` argument, which would automatically compile everything with
the device compiler as well as test the code on all backends.
`ALL_BACKENDS` is still supported, but it no longer changes the sources to
be compiled with the device compiler.
The `RuntimeDeviceTracker` is a thread-local variable that monitors the
devices to use separately on each thread. This is an important feature
to allow different threads to control different devices.
When a tracker is created on a new thread, it was simply reset, which
makes sense. However, the reset does not take into account the device
selected by `vtkm::cont::Initialize`. This means that if VTK-m was used
in a different thread than it was initialized, it would ignore the
`--vtkm-device` parameter.
To get around this problem, keep track of the `RuntimeDeviceTracker` on
the "main" thread. When a `RuntimeDeviceTracker` is created on a new
thread, it copies the state from that tracker.
