Convert lazy SkImages to raster images when extracting pixel data (#5713)

Fixes https://github.com/flutter/flutter/issues/19214

lib/ui/painting/image_encoding.cc

@@ -57,12 +57,23 @@ void InvokeDataCallback(std::unique_ptr<DartPersistentValue> callback,
 
 sk_sp<SkImage> ConvertToRasterImageIfNecessary(sk_sp<SkImage> image,
                                                GrContext* context) {
-  if (context == nullptr) {
-    // The context was null (software rendering contexts) so the image is likely
-    // already a raster image. Nothing more to do.
+  SkPixmap pixmap;
+  if (image->peekPixels(&pixmap)) {
+    // This is already a raster image.
     return image;
   }
 
+  if (sk_sp<SkImage> raster_image = image->makeRasterImage()) {
+    // The image can be converted to a raster image.
+    return raster_image;
+  }
+
+  // Cross-context images do not support makeRasterImage.  Convert these images
+  // by drawing them into a surface.
+  if (context == nullptr) {
+    return nullptr;
+  }
+
   TRACE_EVENT0("flutter", __FUNCTION__);
 
   // Create a GPU surface with the context and then do a device to host copy of