When syncing the texture's sampler state, make sure the correct texture
unit is active.  When StateManagerGL syncs the textures, it may not set
the active texture if the texture ID has not changed.

BUG=angleproject:884
BUG=angleproject:883
BUG=angleproject:1051

Change-Id: Ib22874c22a12b1b9bd68887ac5a449c507e7bdc6
Reviewed-on: https://chromium-review.googlesource.com/298482Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>