gl-renderer: Assume axis alignment using node's valid_transform

Assume axis alignment using node's valid_transform boolean instead of
relying on the transform flags of the weston_view struct. This is more
reliable since matrix flags could erroneously hang around after a
series of transforms. Additionally, nodes with standard output
transforms like translations, flips and rotations by 90° can now take
the fast axis aligned path.

Signed-off-by: Loïc Molinari <loic.molinari@collabora.com>