patch which should add llTextBox() functionality

this patch is based on the patch by Henri for the Singularity viewer (https://github.com/siana/SingularityViewer/commit/3a4093ccc30f07c987896d05a5c2d2620e62d0d0)
I noticed the lsl function "llTextBox()" was not working correctly in the imprudence viewer when I tried implementing it recently in one of my projects, so I thought I would delve into the viewer source code to see why. This should fix, I will try to compile on my own machine in the meantime to verify that it is functional...