bits_image_fetch_pixel_convolution(): Make sure channels are signed

bits_image_fetch_pixel_convolution(): Make sure channels are signed

In the computation:

    srtot += RED_8 (pixel) * f

RED_8 (pixel) is an unsigned quantity, which means the signed filter
coefficient f gets converted to an unsigned integer before the
multiplication. We get away with this because when the 32 bit unsigned
result is converted to int32_t, the correct sign is produced. But if
srtot had been an int64_t, the result would have been a very large
positive number.

Fix this by explicitly casting the channels to int.