Adds obs_add_raw_video_callback() and obs_remove_raw_video_callback()
functions which allow the ability to get raw video frames without
necessarily needing to create an output.
(Note: This commit also modifies UI and test)
This makes it so that main preview panes are rendered with the main
output texture rather than re-rendering the main view. The view will
render all objects again, whereas the output texture will be a single
texture render of the same exact thing.
Also fixes some abnormal artifacting when scaling the main preview pane.
Adds circlebuf_push_front_zero and circlebuf_push_back_zero to
conveniently push zeroed data to the front/back of the buffer without
having to create an intermediary buffer to accomplish the same thing.
