Fix: reset s_renderFrameCalled in bgfx::shutdown() to allow re-init (#3690)

bgfx::renderFrame() sets s_renderFrameCalled = true on its first call
(line 1541) and never clears it. bgfx::shutdown() resets s_threadIndex
to 0 (line 3958) but leaves s_renderFrameCalled sticky, which means
that on a 2nd renderFrame after shutdown the gate at line 1534 still
runs BGFX_CHECK_RENDER_THREAD(). That assertion checks
(s_ctx != NULL && single-threaded) || (~BGFX_API_THREAD_MAGIC == s_threadIndex);
both branches fail post-shutdown (s_ctx is NULL, s_threadIndex is 0),
and the assertion fires in debug builds — blocking re-init of the
bgfx context within the same process.

Adding the symmetric s_renderFrameCalled reset to shutdown() makes
shutdown leave bgfx in the same state as a fresh process, so a
subsequent renderFrame()+init() pair behaves like the first one.

Verified locally: a downstream test that constructs an SDL+bgfx
context, runs a frame, destroys it, then constructs another from the
same thread now passes in debug builds with this patch (it asserts at
line 1536 without it).

src/bgfx.cpp

@@ -3956,6 +3956,7 @@ namespace bgfx
 		}
 
 		s_threadIndex = 0;
+		s_renderFrameCalled = false;
 		g_callback    = NULL;
 		g_allocator   = NULL;
 	}