Ensured ImageView requests inside ResourceReady signal handler are queued.

Ensured ImageView requests inside ResourceReady signal handler are queued.

Deferring ImageView load requests until after ResourceReady signal
handler.  has completed ensures that attempting to re-load images
doesn't fail to send a second ResourceReady callback.

Now also tries to re-load images that had previously failed.

TextureManager::LoadTexture() now checks for deferred loading when
setting 'loading' flag in output parameters. Note, loading an existing
texture may also cause loading flag to get set - this is to ensure
that the caller, e.g. ImageVisual handles the deferred Complete
callback as if it were asynchronous.

An application can still cause an infinite loop if it doesn't have a max
retry count when attempting to re-load failed images inside the signal
handler. This is considered to be an application bug, not a DALi bug.
( Control::ResourceReady signal is not a one-shot signal).

Change-Id: I2c505623ce5e02d3ae67e6e06fd80d5108dc8ade
Signed-off-by: David Steele <david.steele@samsung.com>