textoverlay: add "roll" property

Submitted by César Fabián Orccón Chipana

Created attachment 323210
Allow textoverlaysrc to roll text.

Hello. I have developed this patch during the GNOME & FEDORA HACK CAMP 2016 :)
You can do something like

gst-launch-1.0 videotestsrc ! textoverlay text=whatever roll=true roll-orientation=left roll-speed=60 ! xvimagesink

Enjoy it! :)

Patch 323210, "Allow textoverlaysrc to roll text.":
0001-gstbasetextoverlay-Add-roll-properties.patch