glthread: add support for TexParameteri and SamplerParameteri functions

It's straightfoward except that I had to hack the python scripts to add
"marshal_count", which behaves just like "count" except that "variable_param"
is ignored. ("variable_param" changes the behavior of "count", which I don't
want)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3948>

src/mapi/glapi/gen/gl_and_es_API.xml

@@ -259,7 +259,8 @@
     <function name="TexParameterxv" es1="1.1" desktop="false">
         <param name="target" type="GLenum"/>
         <param name="pname" type="GLenum"/>
-        <param name="params" type="const GLfixed *" variable_param="pname"/>
+        <param name="params" type="const GLfixed *" variable_param="pname"
+               marshal_count="_mesa_tex_param_enum_to_count(pname)"/>
     </function>
 
     <!-- from GL_OES_single_precision -->