stand: Fix shadow buffer offset handling

The shadow buffer is addressed relative to tg_origin, which includes
the padding offset, whereas gfxfb_blt operates on coordinates without
that offset. To make gfx_fb_copy_area emulate the behavior of
gfxfb_blt, the source coordinates must include the padding offset,
while the destination coordinates must not. The original implementation
omitted the offset from the source coordinates; this change corrects
that.

Additionally, gfx_fb_cons_display already applies the padding offset,
so the redundant adjustment is removed.

PR: 296246
Reported by: 2khramtsov@gmail.com
Reviewed by: imp
Tested by: 2khramtsov@gmail.com, junchoon@dec.sakura.ne.jp,
naito.yuichiro_@gmail.com
Fixes: 32da2f23ae4d
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D57821

(cherry picked from commit 76aa776b5f47ecd0d45336e22795fef98af57d2f)