Accumulate subpixel mouse motion so motion is not lost.

Previously when the canvas was scaled up and the pointer was locked,
motion corresponding to less than one pixel was lost. Therefore,
slow mouse motion resulted in no motion. This fixes that.

src/video/emscripten/SDL_emscriptenevents.c

@@ -301,25 +301,33 @@ EM_BOOL
 Emscripten_HandleMouseMove(int eventType, const EmscriptenMouseEvent *mouseEvent, void *userData)
 {
     SDL_WindowData *window_data = userData;
-    int mx = mouseEvent->canvasX, my = mouseEvent->canvasY;
+    int mx, my;
+    static double residualx = 0, residualy = 0;
     EmscriptenPointerlockChangeEvent pointerlock_status;
 
+    /* rescale (in case canvas is being scaled)*/
+    double client_w, client_h, xscale, yscale;
+    emscripten_get_element_css_size(NULL, &client_w, &client_h);
+    xscale = window_data->window->w / (client_w * window_data->pixel_ratio);
+    yscale = window_data->window->h / (client_h * window_data->pixel_ratio);
+
     /* check for pointer lock */
     int isPointerLockSupported = emscripten_get_pointerlock_status(&pointerlock_status);
     int isPointerLocked = isPointerLockSupported == EMSCRIPTEN_RESULT_SUCCESS  ? pointerlock_status.isActive : SDL_FALSE;
 
     if (isPointerLocked) {
-        mx = mouseEvent->movementX;
-        my = mouseEvent->movementY;
+        residualx += mouseEvent->movementX * xscale;
+        residualy += mouseEvent->movementY * yscale;
+        /* Let slow sub-pixel motion accumulate. Don't lose it. */
+        mx = residualx;
+        residualx -= mx;
+        my = residualy;
+        residualy -= my;
+    } else {
+        mx = mouseEvent->canvasX * xscale;
+        my = mouseEvent->canvasY * yscale;
     }
 
-    /* rescale (in case canvas is being scaled)*/
-    double client_w, client_h;
-    emscripten_get_element_css_size(NULL, &client_w, &client_h);
-
-    mx = mx * (window_data->window->w / (client_w * window_data->pixel_ratio));
-    my = my * (window_data->window->h / (client_h * window_data->pixel_ratio));
-
     SDL_SendMouseMotion(window_data->window, 0, isPointerLocked, mx, my);
     return 0;
 }