Fix bug #4681.

Fix bug #4681.

glDeleteTextures and glDeleteTexturesEXT were erroneously listed as
aliases of each other.  For anything /except/ GLX protocol they are
aliases.  This set of changes allows functions that are functionally
identical but have different GLX protocol to be listed as aliases.

When building with GLX_INDIRECT_RENDERING set, different static
functions are used.  These functions determine whether the current
context is direct rendering or not.  If the context is direct
rendering, the aliased function (e.g., glDeleteTextures in the case of
glDeleteTexturesEXT) is called.  If the context is not direct
rendering, the correct GLX protocol is sent.

For a deeper explanation of what is changed, please see:
http://dri.freedesktop.org/wiki/PartiallyAliasedFunctions