rtpdec_jpeg: fix low contrast image on low quality setting

The problem is that the argument 'q' is of the type uint8_t.
According to the JPEG standard, if 1 <= q <= 50, the scale factor
'S' should be 5000 / Q. Because the create_default_qtables() reuses
the variable 'q' to store the result of this calculation, for small
values of q < 19, q wil subsequently overflow and give wrong results
in the calculated quantization tables.

Instead, use a new variable 'S' (same name as in RFC2435) with the
proper range to store the result of the division.

Signed-off-by: Martin Storsjö <martin@martin.st>
