video: Expose dpi and window status getters

These are currently used directly in CVideo::video_settings_report(),
which we want to remove from this class and implement externally.

This commit also reworks get_dpi_scale_factor() to be implemented in
terms of get_dpi().

src/video.cpp

@@ -406,23 +406,25 @@ bool CVideo::window_has_flags(uint32_t flags) const
 	return (window->get_flags() & flags) != 0;
 }
 
-std::pair<float, float> CVideo::get_dpi_scale_factor() const
+std::pair<float, float> CVideo::get_dpi() const
 {
-	std::pair<float, float> result{1.0f, 1.0f};
-
-	if(!window) {
-		return result;
-	}
-
 	float hdpi, vdpi;
-	int returncode = SDL_GetDisplayDPI(window->get_display_index(), nullptr, &hdpi, &vdpi);
-
-	if (returncode == 0) {
-		result.first = hdpi / MAGIC_DPI_SCALE_NUMBER;
-		result.second = vdpi / MAGIC_DPI_SCALE_NUMBER;
+	if(window && SDL_GetDisplayDPI(window->get_display_index(), nullptr, &hdpi, &vdpi) == 0) {
+		return { hdpi, vdpi };
 	}
+	// SDL doesn't know the screen dpi, there's a configuration issue, or we
+	// don't have a window yet.
+	return { 0.0f, 0.0f };
+}
 
-	return result;
+std::pair<float, float> CVideo::get_dpi_scale_factor() const
+{
+	auto dpi = get_dpi();
+	if(dpi.first != 0.0f && dpi.second != 0.0f) {
+		return { dpi.first / MAGIC_DPI_SCALE_NUMBER, dpi.second / MAGIC_DPI_SCALE_NUMBER };
+	}
+	// Assume a scale factor of 1.0 if the screen dpi is currently unknown.
+	return { 1.0f, 1.0f };
 }
 
 std::vector<point> CVideo::get_available_resolutions(const bool include_current)

src/video.hpp

@@ -79,6 +79,11 @@ class CVideo
 	/** Returns a pointer to the underlying SDL window. */
 	sdl::window* get_window();
 
+	bool has_window()
+	{
+		return get_window() != nullptr;
+	}
+
 private:
 	enum MODE_EVENT { TO_RES, TO_FULLSCREEN, TO_WINDOWED, TO_MAXIMIZED_WINDOW };
 
@@ -127,6 +132,9 @@ class CVideo
 	/** Returns the window renderer height in pixels or in screen coordinates. */
 	int get_height(bool as_pixels = true) const;
 
+	/** The current game screen dpi. */
+	std::pair<float, float> get_dpi() const;
+
 	/** The current scale factor on High-DPI screens. */
 	std::pair<float, float> get_dpi_scale_factor() const;