kanjitalk755 x11 SS color channel issues

[rakslice]

When building kanjitalk755's current code a528b0a40f1039208422d35a013adb56de6689d8 with X11 graphics (--enable-sdl-video=no) while testing to work around an SDL2 issue, the resulting graphics had colour channels missing or swapped:

• Expected white backgrounds were yellow indicating the blue channel was possibly missing/zero
• The Apple icon in the Apple Menu had red as its top section, indicating the red and green channels were possibly swapped.

I wonder if this is an endianness issue. This issue didn't affect the earliest default SDL2 version I was testing (52fe2290fe5816ad67aa2796331773c2c5208ee3)

[rakslice]

As noted in the comments in I think in the Basilisk macosx video driver, where the buffer is offset by 1 byte to get ARGB -> RGBA for free, the guest buffer is ARGB.

The behaviour we're seeing is consistent with byte swapped ARGB (R<->G, B<->A).

Indeed, just forcing need_msb_image to true in video_x.cpp makes the problem go away.

[rakslice]

I recently did a test x11 build on Linux to see if I could get DGA full screen mode to work (no luck), but I noticed that I didn't get the colour channel issue. I wonder if this was just a blitting problem connected to #46