mangohud: initialize the window_size

This one seems janky - other frontends (glx/egl LD_PRELOAD, Vulkan)
init their own global window_size.

Although in reality we use the initial data solely to set the imgui
"window" position and we immediatelly overwrite it, with the follow-up
call render_imgui().

Odds are this won't fix the glitchy (re)sizes but it's a step in the
right direction.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

src/app/main.cpp

@@ -266,6 +266,7 @@ int main(int, char**)
     init_cpu_stats(*params);
     notifier.params = params;
     start_notifier(notifier);
+    window_size = ImVec2(params->width, params->height);
         deviceName = (char*)glGetString(GL_RENDERER);
     sw_stats.deviceName = deviceName;
     if (deviceName.find("Radeon") != std::string::npos