samples/hidpi: add live mouse coordinate readout

samples/hidpi.c

@@ -20,6 +20,9 @@
 	  - A live readout of `cf_app_get_pixel_scale()` alongside the physical
 	    canvas size, so the current display's HiDPI scale factor is visible
 	    at a glance.
+	  - A live readout of the mouse position in both raw screen coordinates
+	    and `cf_screen_to_world`'s translation, so the HiDPI mouse mapping
+	    can be eyeballed against known shape positions.
 
 	No interactivity beyond closing the window; no external assets needed.
 */
@@ -92,6 +95,23 @@ int main(int argc, char* argv[])
 		draw_text_centered(pixel_scale_buf, cf_v2(0, 300));
 		cf_pop_font_size();
 
+		// -- Live mouse-coordinate readout -- screen space is raw, top-left-origin,
+		// y-down input; world space is what cf_screen_to_world hands back, matching
+		// the space cf_draw_text etc. draw into. If HiDPI translation is correct, the
+		// world coordinate should track the cursor 1:1 over the canvas regardless of
+		// pixel_scale.
+		char mouse_buf[128];
+		CF_V2 mouse_screen = cf_v2(cf_mouse_x(), cf_mouse_y());
+		CF_V2 mouse_world = cf_screen_to_world(mouse_screen);
+		snprintf(
+			mouse_buf, sizeof(mouse_buf),
+			"mouse: screen (%.0f, %.0f) -> world (%.0f, %.0f)",
+			mouse_screen.x, mouse_screen.y, mouse_world.x, mouse_world.y
+		);
+		cf_push_font_size(12);
+		draw_text_centered(mouse_buf, cf_v2(0, 300 - 18));
+		cf_pop_font_size();
+
 		cf_draw_pop_color();
 
 		cf_draw_push_color(cf_make_color_rgb(120, 130, 150));