ElmerGUI: unable to select multiple surfaces

[agalakhov]

In most recent Git (devel) versions holding Ctrl to select multiple surfaces (i.e. for Unify) either is not reliable or does not work at all.

Steps to reproduce:

• Open "cooler.grd"
• Divide surface
• Hold Ctrl and try selecting surfaces by double-clicking on them

[agalakhov]

I traced this problem down to thread-safety. The variable GLWindet::ctrlPressed is a non-atomic bool, and there is no single memory barrier used in the code. Despite setting it to true in the event handler, it could still be read as false in another event handler if the processor is multi-core and has advanced caching. But changing it to std::atomic_bool is only a partial solution. While I can now select multiple surfaces, the software still behaves in a very odd way. Looks like there are even more race conditions.