Add texture transforms

Texture transforms allow to change the way a texture is mapped on
a surface. They can be used by adding a transform child to a
texture_instance in the XML project file.

Issue #43
Allow a single transform for texture

Project file was missinterpreted. We don't need a transform sequence for
a texture because we don't want texture specific motion blur.
Add test scene for texture transform

Contains appleseed project file, a simple exr texture and a copy of
the texture in png format.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
Project file change for texture transform

This should be in the previous commits, but there are some problems
with the Windows file format on my machine.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>