The imgQ code in RFB should be a generic rendering queue system in
Display.

The reason for the render queue in the first place is that images
loaded from raw data URI strings aren't immediately ready to display
so we have to wait for them to complete 'loading'. However, when data
URI images are mixed with other types of rendering actions then things
can get out of order. This is the reason for the rendering queue.

Currently this only keeps display actions for tight and tightPNG
related actions in order (because they use a mix of fills, raw pixel
data and data URI images).