[cogl] renderer: Don't allow the COGL_DRIVER env var to override application

[Neil Roberts <nroberts src gnome org>]

commit 4e0942d74c8d69aa48e0176bfecf27f64a950d0f
Author: Neil Roberts <neil linux intel com>
Date:   Thu Jun 27 09:57:02 2013 +0100

    renderer: Don't allow the COGL_DRIVER env var to override application
    
    Since e07d0fc7441 the COGL_DRIVER environment variable was able to
    override the application's driver selection. This doesn't seem like a
    good idea because if the application is specifying a driver explicitly
    then presumably it can not work with any other driver. This patch
    changes it so that if a driver is selected in the configuration and by
    the application then they must match, otherwise it will fail with a
    CoglError.
    
    Reviewed-by: Robert Bragg <robert linux intel com>

 cogl/cogl-renderer.c |   18 ++++++++++++++++--
 1 files changed, 16 insertions(+), 2 deletions(-)
---
diff --git a/cogl/cogl-renderer.c b/cogl/cogl-renderer.c
index ecbf8bd..33e086c 100644
--- a/cogl/cogl-renderer.c
+++ b/cogl/cogl-renderer.c
@@ -501,8 +501,22 @@ _cogl_renderer_choose_driver (CoglRenderer *renderer,
       if (driver_override == COGL_DRIVER_ANY)
         invalid_override = driver_name;
     }
-  else
-    driver_override = renderer->driver_override;
+
+  if (renderer->driver_override != COGL_DRIVER_ANY)
+    {
+      if (driver_override != COGL_DRIVER_ANY &&
+          renderer->driver_override != driver_override)
+        {
+          _cogl_set_error (error,
+                           COGL_RENDERER_ERROR,
+                           COGL_RENDERER_ERROR_BAD_CONSTRAINT,
+                           "Application driver selection conflicts with driver "
+                           "specified in configuration");
+          return FALSE;
+        }
+
+      driver_override = renderer->driver_override;
+    }
 
   if (driver_override != COGL_DRIVER_ANY)
     {