[core] Fix assertion at RenderRasterLayer::prepare()

It shall consider that image data might not be available until
the source loads it.

src/mbgl/renderer/render_layer.cpp

@@ -47,6 +47,7 @@ bool RenderLayer::supportsZoom(float zoom) const {
 
 void RenderLayer::prepare(const LayerPrepareParameters& params) {
     assert(params.source);
+    assert(params.source->isEnabled());
     renderTiles = params.source->getRenderTiles();
     addRenderPassesFromTiles();
 }