Expose ImGuiContext API for using across DLL boundaries

[mjakeman]

Exposes a Zig equivalent to SetCurrentContext() + SetAllocatorFunctions() for sharing ImGui state across DLL/library boundaries.

I'm using this as part of a hot-reloading mechanism (which relies on shared libraries).

Relevant part of the ImGui docs:

//-----------------------------------------------------------------------------
// [SECTION] CONTEXT AND MEMORY ALLOCATORS
//-----------------------------------------------------------------------------

// DLL users:
// - Heaps and globals are not shared across DLL boundaries!
// - You will need to call SetCurrentContext() + SetAllocatorFunctions() for each static/DLL boundary you are calling from.
// - Same applies for hot-reloading mechanisms that are reliant on reloading DLL (note that many hot-reloading mechanisms work without DLL).
// - Using Dear ImGui via a shared library is not recommended, because of function call overhead and because we don't guarantee backward nor forward ABI compatibility.
// - Confused? In a debugger: add GImGui to your watch window and notice how its value changes depending on your current location (which DLL boundary you are in).

[hazeycode]

I see this has been copied from init fn but I don't like it. Just making a note.