[IPOL discuss] Fast image loading

[Miguel Colom]

Dear all,
as you may have noticed in the demos, sometimes there's a delay  
between the time you click on an image of the demo and the time the  
parameters page appears.

This is caused by internal format conversions performed by the demo  
system to ensure that the binary program will receive the images in  
the format it needs and also to create versions of the images that can  
be displayed on the web with the proper sizes.

However, most of the time these kind of conversions are not needed.  
For example, if the input image is PNG, it doesn't need to be  
converted again into PNG. Also, most of the these operations can be  
executed in parallel.

So, I did these changes:
- Parallelization of all conversions that can be performed at the same time.
- Only convert input files is necessary. If not, just copy the input  
(after a security check, of course).

Therefore, from now you should observe a drastic increase in the  
images loading speed.
Of course, if conversions are needed (say, the input image is JPEG and  
the binary needs PNG), the process will slow down, but it will be  
parallelized anyway.
In case of PNG images (the format that most of the demos and users  
prefer), the performance is now really improved.

Best,
Miguel

PS: this change has affected files base_app.py and image.py in lib/