Have SkWuffsCodec get workbuf_len from decoder

Getting it from the image_config is deprecated. There may be more than
one implementation of decoding any given compressed image format.

Bug: skia:8235
Change-Id: I17cb746467635c073c5bc55946b2bad4f067b1bf
Reviewed-on: https://skia-review.googlesource.com/c/183540
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>