util: fix pwetty on big-endian

util: fix pwetty on big-endian

Cairo always uses native byte order for rendering.

Hence if the byte order of the frame buffer differs from the byte order
of the CPU, the frame buffer contents need to be byteswapped twice: once
before rendering, to convert to native byte order, and a second time
after rendering, to restore the frame buffer format's byte order.

Note that byte swapping is not done for ARGB32 formats, as for these
formats, byte order only affects the order of the red, green, and blue
channels, which we do not care about here.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
---
v5:
  - Add Reviewed-by,

v4:
  - No changes,

v3:
  - Wrap byteswap_buffer{16,32}() implementation inside #if HAVE_CAIRO
    to avoid defined-but-not-used compiler warnings,

v2:
  - RGB30 is untested.