add FreeTexImageData hook to help single-copy texturing in drivers
diff --git a/src/mesa/main/context.c b/src/mesa/main/context.c
index 69610e4..a1e40e9 100644
--- a/src/mesa/main/context.c
+++ b/src/mesa/main/context.c
@@ -1536,6 +1536,7 @@
 {
    ASSERT(driverContext);
    assert(driverFunctions->NewTextureObject);
+   assert(driverFunctions->FreeTexImageData);
 
    /* If the driver wants core Mesa to use special imports, it'll have to
     * override these defaults.