[ci skip]Texture async loading leak fix

Fix a texture memory leak which occurred if textures which were loading asynchronously in the background were also loaded in the foreground (render/game thread) before the asynchronous loading had finished. The issue was that once asynchronous loading had finished, we weren't checking to see if the texture had already been loaded on a different thread. Fix this issue by checking for this situation and add a test to reproduce the old issue and test the fix.