wgl: Correctly check GLX capabilities. Use client GLX extensions on ATI hardware to work around driver bugs.

For now we forward the methods to the data cache as before.

Add tests for these.

urlmon: The pcbRead parameter for IInternetProtocol::Read is optional, so fix the protocol implementation to check for this.

Add some missing prototypes to mscat.h.
Update win32.api to fix the winapi_check warnings.