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-transitions.cpp

@@ -1222,7 +1222,7 @@ void OBSBasic::RenderProgram(void *data, uint32_t cx, uint32_t cy)
 
 	window->DrawBackdrop(float(ovi.base_width), float(ovi.base_height));
 
-	obs_render_main_view();
+	obs_render_main_texture();
 	gs_load_vertexbuffer(nullptr);
 
 	/* --------------------------------------- */