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

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

When scaling a buffer 2x and calling ply_pixels_interpolate to interpolate
the last row / column, the extra pixels used for pixels would go out of
bounds and be replaced with a black pixel. This causes a 50% dimming of the
last row / column.

This 50% dimming leads to an ugly darkline when a theme draws 2 images
which are supposed to be joined together.

This commit fixes this by clipping the coordinates to the source image
limits instead of using black pixels when interpolating right and bottom
edge pixels.

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