Bug 1573914 - Fix backdrop-filter jitter during animation due to snap…

… offsets r=gw

Since the filtered-backdrop is 1:1 with the backdrop, we don't want any snap offsets to shift the contents

Differential Revision: https://phabricator.services.mozilla.com/D42042

[wrupdater] From https://hg.mozilla.org/mozilla-central/rev/9e513ae4747fad12001ef627e6f273624a3feff4

webrender/src/batch.rs

@@ -2409,7 +2409,7 @@ impl BatchBuilder {
                     local_rect: backdrop_picture.snapped_local_rect,
                     local_clip_rect: prim_info.combined_local_clip_rect,
                     transform_id,
-                    snap_offsets,
+                    snap_offsets: SnapOffsets::empty(),
                     specific_prim_address: prim_cache_address,
                 };