Avoid Resource unnecessarily asking for new data when nothing changed.

This wasn't as simple, turns out I had to fix setFallback() to keep
doing the right thing.

doc/changelog.dox

@@ -280,6 +280,9 @@ See also:
 -   Dynamic plugins on static Magnum builds on Windows were accidentally
     searched for in the `lib/` directory instead of `bin/`, and in some cases
     in `bin/` instead of `lib/` on Unix platforms.
+-   @ref Resource was unnecessarily querying the @ref ResourceManager for
+    updated data even in cases where no resource update was done since last
+    check
 
 @subsection changelog-latest-deprecated Deprecated APIs
 

src/Magnum/Resource.h

@@ -285,7 +285,7 @@ template<class T, class U> void Resource<T, U>::acquire() {
     if(_state == ResourceState::Final) return;
 
     /* Nothing changed since last check */
-    if(_manager->lastChange() < _lastCheck) return;
+    if(_manager->lastChange() <= _lastCheck) return;
 
     /* Acquire new data and save last check time */
     const typename Implementation::ResourceManagerData<T>::Data& d = _manager->data(_key);

src/Magnum/ResourceManager.h

@@ -527,6 +527,9 @@ template<class T> void ResourceManagerData<T>::set(const ResourceKey key, T* con
 template<class T> void ResourceManagerData<T>::setFallback(T* const data) {
     safeDelete(_fallback);
     _fallback = data;
+    /* Notify resources also in this case, as some of them could go from empty
+       to a fallback (or from a fallback to empty) */
+    ++_lastChange;
 }
 
 template<class T> void ResourceManagerData<T>::free() {