Fixed Graphite non-MSAA clipping bug

When the clip atlas cannot accommodate a clip mask, it falls back to a
proxy cache path that creates a standalone texture. This fallback path
called RasterMaskHelper::Allocate() with zero padding, but then set
outPos as if padding existed.

This mismatch caused the translation offset to shift the rendered clip
shape by 1 pixel, leaving the left/top edge undrawn, but the shape
overflowed the right/bottom edge, overwriting those edge pixels with
0x00 ("clipped").

Bug: b/487347682
Change-Id: Iffcb3cb1c93c8537c0c278ce1ca5eb38382807ea
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1203616
Commit-Queue: Thomas Smith <thomsmit@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Thomas Smith <thomsmit@google.com>