Fix #2467 bmp encoding issue for BMP with 1 bit per pixel and more pixels per row than divisible by 8.

[synercoder]

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Description

Fix for issue #2467.

If you have a BMP with 1 bit per pixel, and more pixels per row than divisible by 8, the last byte of the row will need to be padded with extra zeros.

So if you have a black/white image of 10 pixels in a row, and the row looks like:
bwwbbbwbww
The bytes should look like:"
0x01100010 0x11000000

However, the current implementation of Write1BitPixelData will when it reaches the last byte looks like:

if (quantizedPixelRow.Length % 8 != 0)
{
    int startIdx = quantizedPixelRow.Length - 7;
    endIdx = quantizedPixelRow.Length;
    Write1BitPalette(stream, startIdx, endIdx, quantizedPixelRow);
}

So quantizedPixelRow.Length will be 10, resulting in a startIdx of 10-7 = 3
It will then write pixels to the byte starting at index 3 again, meaning that the last bytes looks like:
0x00010110, the last byte will always contain the last 7 pixels, instead of the amount of pixels remaining.

This results in that when the image in opened again, the row bytes looks like:
0x01100010 0x00010110

If you check the images in #2467, you can see that the corrupted image shows column 4 & 5 again in column 9 & 10.

The fix

The fix is to read the last X pixels from the quantizedPixelRow when X equals the remaining pixels instead of always the last 7 pixels. So the startIdx needs to be adjusted:
From:
int startIdx = quantizedPixelRow.Length - 7;
To:
int startIdx = quantizedPixelRow.Length - (quantizedPixelRow.Length % 8);

Small note

I am unsure of my test name, so I named it after my issue. I hope this is fine.

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[JimBobSquarePants]

Legend!

I would have been wasting time looking for issues in the quantizer. Thanks for providing a fix.