now prefering RectangleTextures even for POT textures

Because of Stage3D-internals, POT-Textures use up more memory as
necessary (they always allocate mipmaps, whether you need or not).
RectangleTextures do not suffer from this problem, so it makes sense to
default to RectangleTextures all the time.

The downside: after it has been created, you cannot change the "repeat"
property of a texture any more. The repeat setting must now be chosen
when you create the texture. For this reason, all "Texture.from..."
methods now contain a new "repeat" property.

Thus, this change breaks the API at this point (although I don't expect
the repeat property to be in use very often).