feat(simulation): test mode, --test runs a scenery unattended and exits with the result (eu07/maszyna#541)

eu07 --test -s <scenery> [--test-timeout <s>] runs the scenery in ghostview with
the null renderer (models loaded, no textures), a hidden window, no sound,
network or python, and in fixed steps of 0.03 s of simulation time per frame
(three 10 ms physics steps) as fast as it can. Without -seed it uses a fixed
seed, so two runs give byte-identical physicslogs.

The scenery reports its result to the __test.result memory cell, which the
simulator always creates: value 1 passes once every vehicle has stood still for
1 s, value 0 fails at once. The run ends with exit code
  0 pass, 1 fail,
  2 a train passed a signal at stop (SetVelocity 0 0, as seen in its driver's
    speed table; SetVelocity 0 x on automatic block doesn't count),
  3 timeout (test.timeout, default 3600 s of simulation time),
  4 standstill (every vehicle still for test.standstill s, default 300, with no
    result; a pass with test.pass_on_standstill yes),
  5 error (no -s, the scenery didn't load or has no vehicles, an exception).
The test.* keys go in the scenery's config block; test.fail_if_stop_overran no
only logs a signal passed at danger. Outside the test mode a signal passed at
danger is logged.

The null renderer gets a ui backend which draws nothing, hands out the null
texture and material instead of throwing, and ends the ui frame. A GL context
is still made for the hidden window, without asking for a version, as parts
of the ui create textures with gl calls of their own. No crash dialog or
message box in test mode; the window code no longer needs a monitor.

rendering/nullrenderer.cpp

@@ -1,6 +1,25 @@
 #include "stdafx.h"
 #include "rendering/nullrenderer.h"
 
+#include "application/application.h"
+
+void null_imgui_renderer::BeginFrame()
+{
+    auto &io { ImGui::GetIO() };
+    if( false == io.Fonts->IsBuilt() ) {
+        unsigned char *pixels;
+        int width, height;
+        io.Fonts->GetTexDataAsAlpha8( &pixels, &width, &height );
+    }
+}
+
+bool null_renderer::Render()
+{
+    // the ui frame begun by the application has to be ended, or imgui won't start the next one
+    Application.render_ui();
+    return true;
+}
+
 std::unique_ptr<gfx_renderer> null_renderer::create_func()
 {
     return std::make_unique<null_renderer>();