Mouse coordinates are floating point

You can get sub-pixel mouse coordinates and motion depending on the platform and display scaling.

Fixes https://github.com/libsdl-org/SDL/issues/2999

test/testrelative.c

@@ -23,14 +23,17 @@
 #endif
 
 static SDLTest_CommonState *state;
-int i, done;
-SDL_Rect rect;
-SDL_Event event;
+static int i, done;
+static float mouseX, mouseY;
+static SDL_Rect rect;
+static SDL_Event event;
 
 static void
 DrawRects(SDL_Renderer *renderer)
 {
     SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255);
+    rect.x = (int)mouseX;
+    rect.y = (int)mouseY;
     SDL_RenderFillRect(renderer, &rect);
 }
 
@@ -43,8 +46,8 @@ loop()
         switch (event.type) {
         case SDL_MOUSEMOTION:
         {
-            rect.x += event.motion.xrel;
-            rect.y += event.motion.yrel;
+            mouseX += event.motion.xrel;
+            mouseY += event.motion.yrel;
         } break;
         }
     }