Its not intended that users should use these with any other matrix mode, and
internally we now have the _cogl_current_matrix API if we need to play with
other modes.
If we later add internal flags to CoglMatrix then this code wouldn't
initialize those flags. The ways it's now done adds a redundant copy, but
if that turns out to be something worth optimizing we can look again at
using a cast but adding another way for initializing internal flags.
This is useful because sometimes we need to get the current matrix, which
is too expensive when indirect rendering.
In addition, this virtualization makes it easier to clean up the API in
the future.
