Was a threading issue, uiIdle() being called from 2 threads.
Fixes #418
Fixes #227

This buffer is used for the UI thread, which runs slower than RT.
The buffer can fill very quickly, and it's only cleared during
the main thread.
So, increase this buffer in a reasonable amount to compensate.

This allows to run the new LSP plugin UIs.

Closes #409

Fixes #396

Should help #377 a little

Keep thread-safety disabled (optional), as carla does that internally.
Update test code to check for min/max usage

It's not available on all OSes, and it's better to initialize the
struct with memset anyway

This helps #387, but rule is defined by the plugin.
Later on the user must be able to choose the scale mode
(among other things)