... on perceptual gamma image

When constructing the paint core's paint buffer, in GimpBrushCore
and GimpInk, use the drawable's format as the preferred format in
the call to gimp_layer_mode_get_format(), instead of NULL.

Subsequently, use the paint buffer's format, instead of the source
buffer's format, as the preferred iterator format in
do_layer_blend(), since the iterator format must match the paint
buffer format.