video texture helpers: make clang-tidy happy

clang-tidy's bugprone-unused-return-value warns about unique_ptr
release. This use case is valid, as we keep the memory alive elsewhere.
using std::ignore to make the static analyser happy

Change-Id: I863de45d8f73b339e4d2ed1741cf2796644cd1ff
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>