Add 'virtual_mode' optional argument to setDataListener()

[texodus]

Adds an optional argument virtual_mode to the setDataListener() method.

table.setDataListener(listener, {virtual_mode: "vertical"})

There are four virtual modes supported, including the previous (now default) behavior "both". The three new values each disable one or both virtual scroll axes:

• "both" (default) virtualizes scrolling on both axes.
• "vertical" only virtualizes vertical (y) scrolling.
• "horizontal" only virtualizes horizontal (x) scrolling.
• "none" disable all scroll virtualization.

Note that using a virtual_mode other than the default "both" will render the entire <table> along the non-virtual axis(es), and may cause rendering performance degradation.