Restore sprite load check

This is necessary to ensure that background and fill images are ready
to render.

Fixes #1563

src/mbgl/map/map_context.cpp

@@ -181,7 +181,7 @@ void MapContext::update() {
 
         updateTiles();
 
-        if (style->isLoaded()) {
+        if (style->isLoaded() && resourceLoader->getSprite()->isLoaded()) {
             if (!data.getFullyLoaded()) {
                 data.setFullyLoaded(true);
             }