RasterDecoder: only reset necessary gl state for DoCopySubTexture

Because RasterDecoder::DoCopySubTexture only depends on several pixel
unpack related state.

Bug: 902904
Change-Id: I08fe3996c85818c745605ca6e80c5af8fbf713a7
Reviewed-on: https://chromium-review.googlesource.com/c/1354135
Commit-Queue: Peng Huang <penghuang@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613511}
6 files changed