Fixed compiler warnings about type conversions.

Found by buildbot.

src/render/SDL_render.c

@@ -168,8 +168,8 @@ SDL_RendererEventWatch(void *userdata, SDL_Event *event)
     } else if (event->type == SDL_MOUSEMOTION) {
         SDL_Window *window = SDL_GetWindowFromID(event->motion.windowID);
         if (renderer->logical_w && window == renderer->window) {
-            event->motion.x -= (renderer->viewport.x * renderer->dpi_scale.x);
-            event->motion.y -= (renderer->viewport.y * renderer->dpi_scale.y);
+            event->motion.x -= (int)(renderer->viewport.x * renderer->dpi_scale.x);
+            event->motion.y -= (int)(renderer->viewport.y * renderer->dpi_scale.y);
             event->motion.x = (int)(event->motion.x / (renderer->scale.x * renderer->dpi_scale.x));
             event->motion.y = (int)(event->motion.y / (renderer->scale.y * renderer->dpi_scale.y));
             if (event->motion.xrel > 0) {
@@ -187,8 +187,8 @@ SDL_RendererEventWatch(void *userdata, SDL_Event *event)
                event->type == SDL_MOUSEBUTTONUP) {
         SDL_Window *window = SDL_GetWindowFromID(event->button.windowID);
         if (renderer->logical_w && window == renderer->window) {
-            event->motion.x -= (renderer->viewport.x * renderer->dpi_scale.x);
-            event->motion.y -= (renderer->viewport.y * renderer->dpi_scale.y);
+            event->motion.x -= (int)(renderer->viewport.x * renderer->dpi_scale.x);
+            event->motion.y -= (int)(renderer->viewport.y * renderer->dpi_scale.y);
             event->motion.x = (int)(event->motion.x / (renderer->scale.x * renderer->dpi_scale.x));
             event->motion.y = (int)(event->motion.y / (renderer->scale.y * renderer->dpi_scale.y));
         }