This removes the global legacy state for setting the depth state,
backface culling and global program.

The internal _cogl_push_source wrapper which takes a boolean to set
whether legacy state should be applied has been removed. The normal
cogl_push_source can always be used instead.

The CoglDrawFlag for disable legacy state has been removed. Similarly
the boolean argument to disable legacy state when drawing rectangles
has also been removed.

The legacy state testing in test-backface-culling had already been
partially removed in f951d099

 but the masks were not correctly
updated so it was not testing all of the combinations correctly.

The legacy state testing from test-depth-state has been removed.

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