Add draw_manager::get_frame_length() to get the length of one frame

This will usually be determined by the monitor's refresh rate,
but also is rounded down to provide some leeway. It also limits
according to the draw_delay preference.

The loading screen now uses this to limit event calls.

src/draw_manager.cpp

@@ -18,6 +18,7 @@
 #include "exceptions.hpp"
 #include "log.hpp"
 #include "gui/core/top_level_drawable.hpp"
+#include "preferences/general.hpp"
 #include "sdl/rect.hpp"
 #include "video.hpp"
 
@@ -161,7 +162,7 @@ void sparkle()
 	last_sparkle_ = SDL_GetTicks();
 }
 
-static void wait_for_vsync()
+int get_frame_length()
 {
 	int rr = video::current_refresh_rate();
 	if (rr <= 0) {
@@ -170,7 +171,13 @@ static void wait_for_vsync()
 	}
 	// allow 1ms for general processing
 	int vsync_delay = (1000 / rr) - 1;
-	int time_to_wait = last_sparkle_ + vsync_delay - SDL_GetTicks();
+	// if there's a preferred limit, limit to that
+	return std::clamp(vsync_delay, preferences::draw_delay(), 1000);
+}
+
+static void wait_for_vsync()
+{
+	int time_to_wait = last_sparkle_ + get_frame_length() - SDL_GetTicks();
 	if (time_to_wait > 0) {
 		// delay a maximum of 1 second in case something crazy happens
 		SDL_Delay(std::min(time_to_wait, 1000));

src/draw_manager.hpp

@@ -105,6 +105,13 @@ void invalidate_all();
  */
 void sparkle();
 
+/**
+ * Returns the length of one display frame, in milliseconds.
+ *
+ * This will usually be determined by the active monitor's refresh rate.
+ */
+int get_frame_length();
+
 /** Register a top-level drawable.
  *
  * Registered drawables will be drawn in the order of registration,

src/gui/dialogs/loading_screen.cpp

@@ -139,9 +139,9 @@ void loading_screen::spin()
 
 	// Restrict actual update rate.
 	int elapsed = SDL_GetTicks() - last_spin_;
-	if (elapsed > 10 || elapsed < 0) {
-		events::pump_and_draw();
+	if (elapsed > draw_manager::get_frame_length() || elapsed < 0) {
 		last_spin_ = SDL_GetTicks();
+		events::pump_and_draw();
 	}
 }