wall: fix non-first abscissa of non-8-bit samples

The X and Y values are pixel coordinates in the potentially subsampled plane resolution. This must be converted to an octet offset as the type of the pixel plane pointer is pointer-to-uint8_t.

So the X value must be multiplied by the sample size. The current code was instead rounding the value down to a multiple of the sample size.

Bug reported by Philipp Marek on IRC.