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

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

src/mbgl/renderer/layers/render_raster_layer.cpp

@@ -77,12 +77,14 @@ static std::array<float, 3> spinWeights(float spin) {
 void RenderRasterLayer::prepare(const LayerPrepareParameters& params) {
     renderTiles = params.source->getRenderTiles();
     imageData = params.source->getImageRenderData();
-    assert(renderTiles || imageData);
+    // It is possible image data is not available until the source loads it.
+    assert(renderTiles || imageData || !params.source->isEnabled());
 }
 
 void RenderRasterLayer::render(PaintParameters& parameters) {
-    if (parameters.pass != RenderPass::Translucent)
+    if (parameters.pass != RenderPass::Translucent || (!renderTiles && !imageData)) {
         return;
+    }
     const auto& evaluated = static_cast<const RasterLayerProperties&>(*evaluatedProperties).evaluated;
     RasterProgram::Binders paintAttributeData{ evaluated, 0 };
 

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();
 }