ply-pixel-buffer: Fix bottom edge rendering of scaled buffers

Commit 4e1c00b8 (" ply-pixel-buffer: Fix right and bottom edge rendering of scaled buffers"), which tried to fix a minor rendering issue with the right and bottom edge of scaled images, contains a typo which actually makes things worse :|

When checking if iy exceeds the height of the image, ix gets set to the height instead of iy leading to addressing pass part of the buffer which leads to various rendering artifacts.

This commit fixes the typo and thus also the artifacts.

Fixes: #83 (closed)

Signed-off-by: Hans de Goede hdegoede@redhat.com