wined3d: Option to enable PBUFFER use from registry [1/2]

[Jason Green]

There is code in device.c that uses parts of the GLX_SGIX_pbuffer
extension to allow rendering to offscreen textures, but there has
never been a way to get to this code.  There was a BOOL named
pbuffer_support that was always set to FALSE.  Turning this to TRUE
has enabled at least one app
(http://www.humus.ca/index.php?page=3D&ID=54) to correctly render its
textures.  (You also need to hack the pixel shader version to return
2.0 instead of 1.4 in order to run that app, but that is a different
topic altogether).

I've not noticed any problems with other apps while having this turned
on, but there are comments in the code saying that we may experience
problems with this option until we fully support context switching. 
Plus, there is mention of needing it for some apps at
directxwine.sourceforge.net.

Part 1 of this patch enables an option winecfg on the Graphics Tab to
turn this on (it will stay off by default, so unless the user clicks
this checkbox, this whole patchset is effectively a no-op).

Part 2 of this patch checks for the existence of the GLX_SGIX_pbuffer
extension, and providing that the user has enabled PBuffer support and
the hardware supports it, it will allow that part of the code to
execute.

Both patches provide some minor formatting cleanup and change some
strcmp() calls to strcasecmp() so that we disregard the case of the
registry keys we are using.
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