QNX: Fix clearing transparent buffer

Testing whether alphaBufferSize() != 0 does not work, because when no alpha
channel is present, alphaBufferSize() can return '-1', which will cause
non-transparent windows to be  wrongly cleared and an artifact will appear.

Change-Id: Id9e985f105c0bb302cc6f53960a5dbae2acdb921
Reviewed-by: Frank Osterfeld <frank.osterfeld@kdab.com>
Reviewed-by: Wolfgang Bremer <wbremer@blackberry.com>
Reviewed-by: Sergio Ahumada <sahumada@blackberry.com>
QNX: Fix compilation when OpenGL is not available

Change-Id: I67a1d095fc3efd58e9520c9cb3fad13e04a4d64f
Reviewed-by: Frank Osterfeld <frank.osterfeld@kdab.com>
Reviewed-by: Bernd Weimer <bweimer@blackberry.com>