shaped-texture: Do not apply y-inverts to shape masks

This was introduced by accident in commit 1467b6b02a73bca76
y-inverted textures in combination with shape masks appear to
be only commonly used with EGLstreams. However, as we draw the
shape mask ourselves, we don't want to apply the y-invert to it
as testified by the left over `cogl_pipeline_set_layer_matrix()`.
    
Note that we still allow to apply viemports and buffer transforms,
as the Xwayland mode setting emulation may use it (in fact only
the former, but it probably does not hurt to leave the later as well).

Closes #1792 (closed)