[osg-submissions] osgViewer::PixelBufferWin32 updates

[Colin McDonald]

Hi Robert,

Attached is an updated to osgViewer::PixelBufferWin32.

The win32 pbuffer implementation returned an error unless both the
WGL_ARB_pbuffer and the WGL_ARB_render_texture functions were present.
This was too restrictive, as a pbuffer can usefully be created without
render-to-texture, e.g. for use with glReadPixels.  The osg 1.2/Producer
pbuffers worked without RTT, and osgUtil::RenderStage has all the code to
handle both RTT and non-RTT pbuffers, doing a read and copy in the
latter case.

With these changes I have successfully tested the osgprerender example
on a graphics card which supports RTT, and one which doesn't.  Plus
tested in my own application.

In order to aid diagnostics I have also added more function status
return checks, and associated error messages.  I have included the win32
error text in all error messages output.  And there were some errors
with multi-threaded handling of "bind to texture" and a temporary window
context which I have corrected.

These is one (pre-existing) problem with multi-threaded use of pbuffers
in osgViewer & osgprerender, which I have not been able to fix.  A win32
device context (HDC) can only be destroyed from the thread that created
it.  The pbuffers for pre-render cameras are created in
osgUtil::RenderStage::runCameraSetUp, from the draw thread.  But
closeImplementation is normally invoked from the destructor in the main
application thread.  With the additional error messages I have added,
osgprerender will now output a couple of warnings from
osgViewer::PixelBufferWin32::closeImplementation() at exit, after
running multi-threaded on windows.  I think that is a good thing, to
highlight the problem.  I looked into fixing it in osgViewer::Renderer &
osgUtil::RenderStage, but it was too involved for me.  My own
application requirements are only single-threaded.

Unrelated fix - an uninitialised variable in
osg::GraphicsThread::FlushDeletedGLObjectsOperation().

Regards

Colin McDonald

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