Do not convert RGB buffer at once when stride does not fit exact samples.

Do not convert RGB buffer at once when stride does not fit exact samples.

When converting RGB format to RGB format with the same bits per sample,
unscaled path performs conversion on the whole buffer at once. For
non-multiple-of-16 BGR24 to RGB24 conversion it means that padding at the
end of line will be converted too. Since it may be of arbitrary length
(e.g. 8 bytes), operating on the whole buffer produces obviously wrong
results.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>