grayscale: Reduce the scan line a little for BW8

[sjlongland]

Testing with QSSTV with slant-correction disabled, we achieve a more reliable decode if we turn the scan line duration down a wee bit.

The original image (created in Inkscape):

Before modifications:

After:

[sjlongland]

One thing I observe is that QSSTV and slowrx disagree on timings… if I get it working right in one, the slant in the other is off.

Propagation isn't with me tonight but I'll try again and see if I can hit some SSTV cams that are using RXSSTV / MMSSTV. I'll go with the ⅔ majority.

[sjlongland]

Managed to fix decode on the slowrx side using a reference Robot8BW transmission: windytan/slowrx#13

[sjlongland]

Ahh, I'm a goose, I had LSB not USB, that's why the SSTV cams weren't picking it up.

Here's a decode from RXSSTV (in Tasmania) of the signal with corrected timings.