(API Change) libobs: Remove main window funcs/vars

(Non-compiling commit: windowless-context branch)

API Changed:
---------------------
Removed functions:
- obs_add_draw_callback
- obs_remove_draw_callback
- obs_resize
- obs_preview_set_enabled
- obs_preview_enabled

Removed member variables from struct obs_video_info:
- window_width
- window_height
- window

Summary:
---------------------
Changes the core libobs API to not be dependent upon a main window/view.
If you wish to draw to a window/view, use an obs_display object to
handle it.

This allows the use of libobs without requiring a window to be present
on the system.  This is also prunes code that had to be needlessly
duplicated to handle the "main" window.

libobs/obs-video.c

@@ -113,9 +113,6 @@ static inline void render_displays(void)
 
 	pthread_mutex_unlock(&obs->data.displays_mutex);
 
-	/* render main display */
-	render_display(&obs->video.main_display);
-
 	gs_leave_context();
 }