Add case necessary on 32-bit arm builds. (flutter#79)

Offset is a uint64_t that needs a cast.

impeller/renderer/backend/metal/render_pass_mtl.mm

@@ -243,7 +243,7 @@ bool SetBuffer(ShaderStage stage,
       }
       return true;
     }
-    buffers_map[index] = {buffer, offset};
+    buffers_map[index] = {buffer, static_cast<size_t>(offset)};
     switch (stage) {
       case ShaderStage::kVertex:
         [encoder_ setVertexBuffer:buffer offset:offset atIndex:index];