Fix right padding calculation for horizontal centering

[kquick]

The calculations for right padding on horizontal centering don't seem right:

If availWidth is 30, and rWidth is 10, then leftPaddingAmount will be 10. From this, remainder will be 10 and rightPaddingAmount will be 20, which is too large, giving a total width of 10 + 10 + 20 = 40.

Similarly, if availWidth is 30 and rWidth is 11, then leftPaddingAmount will be 8. From this, remainder will be 14, and rPaddingAmount will be 22, giving a total width of 8 + 11 + 22 = 41.

This patch provides a new calculation for rPaddingAmount, which will be 10 and 12, respectively, both giving a resulting length of 30 which matches availWidth.

This does not produce any visible differences on rendering simple scenarios because vty will apply cropping to the Image generated, but it may provide invalid width values for general calculations if there is a Widget to the left or right of the centered Widget.

It may be that vCenterWith has the same type of error; I did not investigate that function in detail.

[jtdaugherty]

Looking at the code, I think the fix is even smaller than your patch. The value remainder was intended only to capture 0 or 1 columns left over due to the amount of padding (left + right) not being evenly divisible by 2. So the fix is

-               remainder = max 0 $ c^.availWidthL - (leftPaddingAmount * 2)
+               remainder = max 0 $ c^.availWidthL - (rWidth + (leftPaddingAmount * 2))

which makes the calculation for rightPaddingAmount work out the way I intended: if the left padding times 2 plus rWidth leaves a "remainder" of 1 column, that needs to be added to the right padding.

I'm really surprised I haven't seen this misbehave before!

[jtdaugherty]

Whoops, pushed the patch to the wrong branch. I just cherry-picked it to master.