Abandon offset support with texel buffers

We don't seem to require nonzero offsets for texel buffers at this
point in time, and requiring this feature greatly reduces the number
of desktop clients that can use texel buffers. If we find a use for
offsets later we can always add it back as a separate feature.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2036953002

Review-Url: https://codereview.chromium.org/2036953002
5 files changed