mosaic.c: If we're not using bluescreen stuff (which needs per pixel alpha values), then use I420 instead of YUVA when resizing the images. (see previous commit for explanation.)

Use I420 image buffers instead of YUVA for mosaic bridge. Impact on CPU usage for test case (http://people.videolan.org/~dionoea/mosaic/ basically a 6x5 mosaic displayed locally) is 155% -> 85% if alpha == 255, 166% -> 110% if alpha == 128.

also scale I420/YV12. (untested)  (btw, why don't we use the "normal" scaling modules for sub pictures?)

* Untested YV12/I420 -> * blending. Works exactly like YUVA blending except when i_trans == 255. This should improve Mosaic performance. (still needs 1 or 2 changes to the mosaic_bridge and mosaic modules).
* Don't try blending if i_trans == 0.

Qt4 - try to provide the missing dialogs for the dialog provider. Should fix skins2 problem on windows.

Mac OS X gui: Label the option "Check for VLC update automatically" rather than "Check for update when VLC is launched".

Mac OS X gui: Ask the user if she wants to check update on startup. And if so, check for update only every two days.

Allow changing the subpicture filter chain at runtime. (This currently kind of segfaults some of the time (after 2 changes) ... somemore bughunting is needed)

Add calls to config_Put* to make sure that the configuration values survive a filter instance destruction.