Implement clipRect with a transform, clipRegion & clipPath

Bug #7146141

When non-rectangular clipping occurs in a layer the render buffer
used as the stencil buffer is not cached. If this happens on a
View's hardware layer the render buffer will live for as long
as the layer is bound to the view. When a stencil buffer is
required because of a call to Canvas.saveLayer() it will be allocated
on every frame. A future change will address this problem.

If "show GPU overdraw" is enabled, non-rectangular clips are not
supported anymore and we fall back to rectangular clips instead.
This is a limitation imposed by OpenGL ES that cannot be worked
around at this time.

This change also improves the Matrix4 implementation to easily
detect when a rect remains a rect after transform.

Change-Id: I0e69fb901792d38bc0c4ca1bf9fdb02d7db415b9