do not assert for slow alpha blending if mirroring is used (tdf#136223)

The blend calls obviously will be skipped if mirroring is involved.
Given how long it took for somebody to notice, this is probably a rare
case, so just alter the assert, until this functionality is actually
needed to be fast.

Change-Id: If1f558cd4a7aac2bb52b677b5901585d878c6543
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/103414
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

0 files changed, 0 insertions, 0 deletions