simplify/cleanup readbuffer impl

- rely more on readPad32
- try to use signed or unsigned read-int correctly

Bug: skia:
Change-Id: Idd9c9b52f54cb47717dbba0d6da654419b3466e8
Reviewed-on: https://skia-review.googlesource.com/89902
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
2 files changed