vulkan: improve EtcToBc transcoding precision. Bug: b/262352590 Change-Id: I6b127acdec28053423423f64ebc6e8561614f0ab Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4290347 Reviewed-by: Yuxin Hu <yuxinhu@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Hailin Zhang <hailinzhang@google.com>