This allows real-time threads to join an audio workgroup on Apple platforms.

float comparison

This patch also updates the MidiDemo to automatically refresh the device
lists when the set of available devices changes.

The issue affected projects not depending on juce_audio_devices.

Previously, whenever the output device sample time changed from
'invalid' to 'valid', the AudioBuffer fifo in the AudioIODeviceCombiner
was cleared. This caused a data race, since the clear operation was not
mutually exclusive with writes from the input device.

This change causes the AudioIODeviceCombiner to keep track of the
timestamp of the first input device callback after the output device is
invalidated. The output device is unable to read from the fifo until its
timestamp exceeds the stored input device callback timestamp.

Previously, JUCE_ANDROID_REALTIME_THREAD_AVAILABLE sometimes remained
unset, even though JUCE_USE_ANDROID_OBOE was set to its default value.
This is because in this case, JUCE_USE_ANDROID_OBOE is only defined in
juce_audio_basics.h, so it is not visible when compiling juce_core.cpp.

Prior to this commit it was not possible to deactivate the first few
channels of an audio device. If say channel 5 was active in the
requested configuration then all channels up to 5 would be active as
well regardless of the provided activation pattern.

As opposed to the number of hardware output channels.