Simplify ImageBuffer conversion using RGBA8888 formats

Using the RGBA8888 formats introduced in Qt 5.2, we can remove the
corresponding code in WebKit. This greatly simplifies the code.

Change-Id: I3df43995c4df53610fc881c68998188ec08370f9
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>
Reviewed-by: Michael Bruning <michael.bruning@digia.com>