st/mesa: trivially enable GL_ATI_texture_env_combine3

This extension is implemented in the texenv program.
Gallium drivers pass patched glean/texCombine.
(I am going to send the patch soon)

Catalyst9.3 advertises this extension too so I don't see a reason we shouldn't.

src/mesa/state_tracker/st_extensions.c

@@ -219,6 +219,8 @@ void st_init_extensions(struct st_context *st)
 
    ctx->Extensions.APPLE_vertex_array_object = GL_TRUE;
 
+   ctx->Extensions.ATI_texture_env_combine3 = GL_TRUE;
+
    ctx->Extensions.MESA_pack_invert = GL_TRUE;
 
    ctx->Extensions.NV_blend_square = GL_TRUE;