Increased table maximum column count from 12 to 20. Fixed table row height display problems.

I guess I should have explained a little more about this. I was having problems getting tables with more than a couple of rows to display correctly. The table grid was not drawn the same as the text, so it would be out of alignment. It also was not shading the header correctly and the X-position of the columns was slightly off. I think I was able to fix most of the display problems with the table.

The increase of column count maximum from 12 to 20 is unrelated to the display problems above, was just due to my need to be able to show more columns.

I just noticed that a similar fix for table layout issues has already been done in the 2.0 branch of the code. So in that case, it probably does not make sense to include this patch at this point. (Although I still would appreciate the maximum column limit to be increased to allow more than 12 column).