Add progress report to tiles loading

[mnaydenov]

Add a no-brainer implementation of the progress report, based on the suggestion in #161
Currently only to the non-threaded implementation.

[mnaydenov]

@farindk Please consider changing the return type of on_progress and start_progress to "bool" so that one can implement cancelation support. Considering these functions are not used at the moment, this will hardly be a breaking change. If this is considered unacceptable, adding on_progress2 , or is_canceled as a separate function could be an option.
Progress reporting without the ability to cancel is needlessly handicapped.

[bobfriesenhahn]

As the developer of GraphicsMagick (http://www.graphicsmagick.org/) I would like to see this implemented once the progress monitor actually does something. I am sure that ImageMagick would use it as well.

[bobfriesenhahn]

I used https://grep.app/ to check for how the decode options passed to heif_decode_image() are used. I did not find any free software which currently attempts to use the progress monitor feature. I have implemented such code for GraphicsMagick but will disable it.

Since there are no apparent users, it should be reasonable to change the function signature.