Using bgfx in a hotloadable dll via setting context

[ddengster]

In Dear IMGUI (and I believe some select stb libraries even have this), there is a concept that you're able to create contexts and pass them across dll boundaries. This enables hotcode reloading and is a huge productivity boost.

See:
https://github.com/ocornut/imgui/blob/37a07858a913be5b4e932fcb38857566f24ff47a/imgui.h#L280
https://github.com/ocornut/imgui/blob/37a07858a913be5b4e932fcb38857566f24ff47a/imgui.cpp#L3692

If you use bgfx code linked in as a hotloadable* dll, the globals here (https://github.com/bkaradzic/bgfx/blob/master/src/bgfx.cpp#L308) would be undefined(or set to 0 in debug mode), rendering them unusable for hotloading. The way to fix this is that we put all the globals into a single context and allow set/getBgfxContext.

This might disrupt some initialization code, but I believe the benefits outweight the costs.

PS. Note that this is NOT the same as building bgfx as a dll and using it wholesale:
#551

Edit: Kind of got this working; here's a sample of the globals you'd need to fix up to use a single (super)context

        void setContext(void* ctx)
	{
		auto superctx = (SuperContext*)ctx;
		s_callbackStub = superctx->callbackStub;
		s_allocatorStub = superctx->allocatorStub;
		g_callback = superctx->callback;
		s_graphicsDebuggerPresent = superctx->graphicsDebuggerPresent;
		g_allocator = superctx->allocator;
		g_caps = superctx->caps;

		s_ctx = superctx->context;
		s_renderFrameCalled = superctx->renderFrameCalled;
		g_internalData = superctx->internalData;
		g_platformData = superctx->platformData;
		g_platformDataChangedSinceReset = superctx->platformDataChangedSinceReset;
		s_threadIndex = superctx->threadIndex;
	}