test-sub-texture: Request texture data in premult format

When requesting the texture data to test that we get back what we
uploaded, we need to ask for it in a premult format otherwise it will
get converted and the test will fail. This was working for the GL
driver because of a bug where it would fail to do the conversion.

Reviewed-by: Robert Bragg <robert@linux.intel.com>

tests/conform/test-sub-texture.c

@@ -271,7 +271,7 @@ validate_result (TestState *state)
   /* Get the texture data */
   p = texture_data = g_malloc (256 * 256 * 4);
   cogl_texture_get_data (COGL_TEXTURE (test_tex),
-                         COGL_PIXEL_FORMAT_RGBA_8888,
+                         COGL_PIXEL_FORMAT_RGBA_8888_PRE,
                          256 * 4, texture_data);
 
   /* Verify the texture data */