mesa: improve getprocaddress test

mesa: improve getprocaddress test

- Allow the getprocaddress test to test extensions not supported by
  Mesa.  The original getprocaddress.py script only included OpenGL
  extension functions that were in Mesa dispatch tables.  Now all
  known extension functions (as detailed in gl_API.xml) are included.
  As the test does not link against any extension function symbols
  (i.e. it uses glXGetProcAddress() for all extension functions),
  it still compiles and links against Mesa; but now the same
  binary can be used to test extensions not yet supported by Mesa.

- Extend the list of tested extension functions.  The last revision
  of this test exercised 16 extension functions; this revision adds
  support for 95 more.