Adding Async remote loading to ImageVisual

Implemented a cut down version of TextureManager to handle Async loading for
local and remote images, without atlasing or handling for broken image.

Modified test harness to manage multiple event triggers using a round-robin
semaphore test. (Required, as we now have 2 AsyncImageLoader threads in
TextureManager, which each generate an EventThreadTrigger).

Change-Id: I06fec6109985c7458a9135f7c3ef85a3f4503c7a
Signed-off-by: David Steele <david.steele@samsung.com>