decoder.decode() was defering close of lzw.decoders created for each
frame in a loop, thus increasing heap usage (referenced object + defered
function) until decode() returns. Memory increased proportionally to the
number of frames. Fix this by moving the sImageDescriptor case block
into its own method.

Fixes #22237

Change-Id: I819617ea7e539e13c04bc11112f339645391ddb9
Reviewed-on: https://go-review.googlesource.com/70370
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>