Cleanups and bugfixes caught by building with -Wall

Compiling with -Wall -Werror is the Right Thing for building C++ with GCC. Unfortunately, turning on -Wall  is super noisy, so -Werror can't be used.

These patches either silence spurious warnings or fix the bugs caught by the warning. There are some warnings that I didn't fix up, because I can't tell if they're bugs or not:

- FPGAThread::alignBuffer(int): should this return return_code?
- EditorViewPort::paint(juce::Graphics&): should this use drawImage() instead?

I also left all of the warnings emitted for Juce's code untouched. I think it's a really bad idea to keep patching up the Juce fork in this tree; it's only going to make it harder to update to a new version later. The right thing to do is to link against a separately built static Juce library.

Anyway, other than the above notes, open-ephys now builds clean with -Wall on linux64.