render: Fix rendering of TAL_GAP rules.

render: Fix rendering of TAL_GAP rules.

A rule that is set to TAL_GAP is supposed to have the same width or
height as a rule of type TAL_1, but without drawing the line.  That
is, it is supposed to be a small blank space between rows or columns.

Unfortunately, TAL_GAP was not implemented properly in the rendering
code.  It was treated just like TAL_0, which meant that it was ignored
and no gap appeared.

This commit implements TAL_GAP, fixing the problem.