WGL: add extension checking

[Roderick Colenbrander]

Hi,

This is a part of a patch which I sent before which rewrote our wglGetProcAddress code and extension loading code. This patch only contains the new extension loading code.

Before all OpenGL extensions were loaded using glXGetProcAddress. The returned function pointers were stored in a table. The function wglGetProcAddress then returned a wrapper extension function which called the stored function pointer.

The issue is that glXGetProcAddress isn't guaranteed to return NULL when an extension isn't found. This patch makes our extension loading code more reliable by checking whether the extension required by the function is really available before calling the display driver to look up a function.

This change required some small changes to the make_opengl32 script which generates all opengl wrapping code. The table with GL extensions has been extended with an extension name field and WGL extensions have been added to it aswell. Before there was a 'glx_name' field aswell which was used for an Nvidia specific GL extension which added the WGL functions wglAllocateMemoryNV / wglFreeMemoryNV. These functions will soon be handled added to winex11.drv as extensions, we won't directly look up the GLX functions from opengl32.dll anymore.

Regards,
Roderick Colenbrander
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