Core: Implement automatic symbol demangling

EGL is the de-facto GL context initialization API, including on X11
where it provides many additional features over GLX.

I’m planning on adding support for selecting the GPU (adapter in
Dolphin-speak) also to OpenGL, similarly to the Vulkan backend, and that
will require EGL, so let’s remove the legacy API first.

This is a very small libary, and as I understand it, it was more or less
developed for Dolphin.

This moves the two relevant files from Externals to Common, changes the
namespace to Common, reformats the code, and adds Dolphin copyright
notices. The change in copyright notice and license was approved by
AdmiralCurtiss.

State: Simplify interthread communication and general cleanups.

CMake: Various improvements

Common: Replace Result with std::expected.