[AD] windowed mode patch

["Isaac Cruz" <icruzbal@xxxxxxxxxx>]

changes in this patch:

- new global variable, desktop_color_depth (only in Windows code)
- new internal variable, dd_pixelformat (of type LPDDPIXELFORMAT) which points
to the current pixel format used (only for software conversion in windowed mode)
- fixed several bugs with system_bitmap (create_system_bitmap always returned
error, and stack overflow in destroying), and made blits with system bitmaps use
DirectX (about 3x faster in my machine)
- implemented a MMX routine to use different color depth as the desktop in
windowed mode (so far, I have written only 32 bit to 16 bit conversion)

Please, try the software conversion (desktop 16 bit and allegro 32 bit) and give
me your opinion. I think I could implement a dirty mechanism, but in what
functions should I check changes in offscreen buffer? I think in locking and in
accelerated functions.

Attachment: win.zip
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