Clear offscreen in nearest_half_pixel_image to white for 565
Clear offscreen in nearest_half_pixel_image to white for 565

Otherwise the offscreen is opaque black

Bug: skia:13752
Change-Id: I24d069ea7cbbac655cb17af26dabf3fab5750549
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/599798
Auto-Submit: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>
Commit-Queue: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@google.com>
Reviewed-by: Florin Malita <a8262bf13ea8c26821e91ea9bca6740a77c96dcd@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia