mesa: Allow generic attributes for glGetActiveAttrib and GL_ACTIVE_ATTRIBUTES

mesa: Allow generic attributes for glGetActiveAttrib and GL_ACTIVE_ATTRIBUTES

Page 77 (page 91 of the PDF) says about glGetActiveAttrib:

    "The returned attribute name can be the name of a generic
    attribute or a conventional attribute (which begin with the prefix
    "gl_", see the OpenGL Shading Language specification for a
    complete list)."

Page 261 (page 275 of the PDF) says about glGetProgramiv:

    "If pname is ACTIVE_ATTRIBUTES, the number of active attributes in
    program is returned."

It doesn't say anything about built-in vs. user-defined attributes.
From the language around glGetActiveAttrib and the lack of an
exclusion of built-in attributes, which exists other places (e.g.,
around glBindAttribLocation), we can infer that GL_ACTIVE_ATTRIBUTES
should include the active attribute count.  It should also be included
in the values returned by glGetActiveAttrib.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=43138
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Tested-by: Yi Sun <yi.sun@intel.com>