Make QWebpHandler::write() work on big endian systems

Currently, we convert image to Format_ARGB32. This means that on big
endian systems the order of bytes will be (0xAA, 0xRR, 0xGG, 0xBB).
However, in WebP library there is no function to import bytes in
ARGB format.

This commit makes us use Format_RGBA8888 and WebPPictureImportRGBA
on big endian systems, which will make the function work correctly.

Change-Id: I5f347f3ec66f0a57df0d40d4900b1573f9400506
Reviewed-by: Lisandro Damián Nicanor Pérez Meyer <perezmeyer@gmail.com>
Reviewed-by: aavit <eirik.aavitsland@digia.com>