- Column would be zero length if there are no visible
pixels in it.
- Trying to draw such a column results in a negative
heightmask in R_DrawColumnTemplate and a probable read
out of bounds.
- In R_DrawColumnTemplate, texheight is used to switch
between a non-PO2 rasterizer and a more efficient PO2
rasterizer.
- There is bounds checking on the non-PO2 version (in the
form of sourcelength) but not on the PO2 version.
- texheight was set to the sprite patch height, which may
be taller than the column (sourcelength), leading to a
read out of bounds.
