Switched engine to use buffer collection.

The old way of allocating images meant that one would have to make
sure that Scenic and Flutter were using exactly the same pixel
formats. This patch removes the old image allocation and replaces
it with a sysmem API that uses buffer collections instead. This
permits a smooth negotiation of formats between the two systems.

Author: dragostis

shell/platform/fuchsia/flutter/compositor_context.cc

@@ -93,7 +93,7 @@ class ScopedFrame final : public flutter::CompositorContext::ScopedFrame {
                  "of size: "
               << physical_size.width() << "x" << physical_size.height();
         } else {
-          task.material.SetTexture(*(surface->GetImage()));
+          task.material.SetTexture(surface->GetImageId());
         }
 
         frame_surfaces.emplace_back(std::move(surface));

shell/platform/fuchsia/flutter/fuchsia_external_view_embedder.cc

@@ -299,8 +299,8 @@ void FuchsiaExternalViewEmbedder::SubmitFrame(
       if (layer->second.canvas_spy->DidDrawIntoCanvas()) {
         const auto& surface_index = frame_surface_indices.find(layer_id);
         FML_DCHECK(surface_index != frame_surface_indices.end());
-        scenic::Image* surface_image =
-            frame_surfaces[surface_index->second]->GetImage();
+        uint32_t surface_image_id =
+            frame_surfaces[surface_index->second]->GetImageId();
 
         // Create a new layer if needed for the surface.
         FML_DCHECK(scenic_layer_index <= scenic_layers_.size());
@@ -363,7 +363,7 @@ void FuchsiaExternalViewEmbedder::SubmitFrame(
             layer->second.surface_size.height() * 0.5f, -layer_elevation);
         scenic_layer.material.SetColor(SK_AlphaOPAQUE, SK_AlphaOPAQUE,
                                        SK_AlphaOPAQUE, SK_AlphaOPAQUE - 1);
-        scenic_layer.material.SetTexture(*surface_image);
+        scenic_layer.material.SetTexture(surface_image_id);
 
         // Only the first (i.e. the bottom-most) layer should receive input.
         // TODO: Workaround for invisible overlays stealing input. Remove when