image before sending it to the mosaic.
                    - Add option ("chroma") to force the image chroma
                    - Remove alpha mask code
 * mosaic.c: - Remove bluescreen code
             - Misc cosmetics changes
 * bluescreen.c: New bluescreen filter (mostly cut & paste from mosaic.c)
 * alphamask.c: New alpha mask filter (mostly cut & paste from mosaic_bridge.c)
 * invert.c: don't invert the alpha plane for YUVA images (We also need to
             prevent that for RGBA images ... but that's not really used
             currently)
 * configure.ac, video_filter/Modules.am: add alphamask and bluescreen