Fix lag when window tab is open on play mode start #33
Description
The on load scene call must be running even when the tab is unfocused while entering playmode, so it would be good to stop while entering since it causes all of the thumbnails to generate again. Either detect if the tab is unfocused or detect if we're in playmode during the hook in order to stop the asset from updating. If the user needs runtime scene objects to be updated, the checks are still performed during processing and the user can manually regenerate thumbnail images from the right click menu.