The image engine is depth aware when using tile drawing the depth is
only updated for the central image what lead to showing the background
on top of other areas.
Also makes sure that switching the tile drawing would lead to an update
of the texture slots.
Previously we used to cache a float image representation of the image in
rect_float. This adds some incorrect behavior as many areas only expect
one of these buffers to be used.
This patch stores float buffers inside the image engine. This is done per
instance. In the future we should consider making a global cache.
Previous implementation had a copy of the image user, which doesn't
contain all the data to identify changes. This patch introduces a new
struct to store the data and can be extended with other data as well
(color spaces, alpha settings).
