tst_QGL::clipTest() fails because QNX graphics drivers don't preserve bits in the back buffer

tst_QGL produces many clipTest failures because the QNX graphics drivers don't preserve bits when a buffer swap occurs (EGL_SWAP_BEHAVIOR).

1) Create a developer build of Qt with tests.
2) From the qtbase/tests/auto/opengl/qgl directory, run "./tst_qgl clipTest".

Many test failures that look like the following will occur:
FAIL! : tst_QGL::clipTest() Pixel Max fuzz (1) exceeded: (0,0,0,255) vs (255,255,255,255)
Loc: [tst_qgl.cpp(1795)]

I think it's because the back buffer is uninitialized. The back buffer doesn't start from a copy of what's in the front buffer. The application draws once, swaps buffers and then compares. What it's comparing to is...empty.

Not sure whether this a problem with the test, QNX failing to live up to a Qt expectation or Qt failing to deal with the lack of preservation.