Micro-optimization for loading 3-channel images.
Micro-optimization for loading 3-channel images.

This reduced the total time spent in LoadToNative3To4 by 10% in our
micro-benchmark.

Test: passed the CQ dry run.
Bug: b/182513949
Change-Id: I5d1b1975f64e5b08bd0e65abc8f94fe62ef54321
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2752347
Commit-Queue: Pujun Lun <44f73f20486fefa5f75e126d2918ae0dbc923dd5@google.com>
Commit-Queue: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Reviewed-by: Shahbaz Youssefi <582eeec4f08a2ccd61e3ef52115e6e7c571582de@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle