It replaces the "fake" demuxer and decoder, and partially
the mjpeg one.
It does not rely on the extensions to detect the format.
The detected picture formats are: XCF, GIF, PNG, TIFF, BMP, PCX,
LBM, PNM, JFIF and SPIFF(Jpeg).
The reload feature of the fake:// demuxer is lost, as well as the deinterlace
feature of the fake decoder, patches are welcome.

It closes #3107 and #3388, and modifies #2295.