libobs: Add obs_render_main_texture

(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.

UI/window-basic-main.cpp

@@ -2968,7 +2968,7 @@ void OBSBasic::RenderMain(void *data, uint32_t cx, uint32_t cy)
 		if (source)
 			obs_source_video_render(source);
 	} else {
-		obs_render_main_view();
+		obs_render_main_texture();
 	}
 	gs_load_vertexbuffer(nullptr);