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.

test/win/test.cpp

@@ -150,7 +150,7 @@ static HWND CreateTestWindow(HINSTANCE instance)
 
 static void RenderWindow(void *data, uint32_t cx, uint32_t cy)
 {
-	obs_render_main_view();
+	obs_render_main_texture();
 
 	UNUSED_PARAMETER(data);
 	UNUSED_PARAMETER(cx);