Ensure pixel buffer is filled even when no callbacks have been registered.

Fixes #17
Re-use existing CCImage when image data is set with filename already handled.

As part of the image caching also some code deduplication and simplification.
Use Filename of EnvironmentTextureNode and ImageTextureNode when it has been set.
Remove irrelevant todo comments.
Add C API for setting vertex normals to a mesh.
Wrap C API for setting vertex normals.
C API: set cast_shadow.
Add vector multiplication.
CSycles: wrap C API set cast_shadow
CCycles: Add support for light falloff node.
CSycles: Add LightFalloffNode

Resolves #18
Move node graph reading code into utilities class.

Move some XML parsing capabilities in Utilities as well. This is
in preparation of a creator function for shaders that can
populate a shader node graph from an XML fragment. The fragment contains
declarations for shader nodes and their connections.

A fragment looks like:

	<texture_coordinate name="texcoord" />
	<brick_texture name="bricktex" scale="2.0" color1="0.8, 0.8, 0.8" color2="1.0, 0.2, 0.2" />
	<background name="bg" strength="1.0" color="0.2, 0.2, 0.2" />
	<connect from="texcoord camera" to="bricktex vector" />
	<connect from="bricktex color" to="bg color" />
	<connect from="bg background" to="output surface" />
Shader from XML utility function.

A shader will be populated with shader nodes and their connections
as presented by the XML fragment passed.

Example fragment:

	<texture_coordinate name="texcoord" />
	<brick_texture name="bricktex" scale="2.0" color1="0.8, 0.8, 0.8" color2="1.0, 0.2, 0.2" />
	<background name="bg" strength="1.0" color="0.2, 0.2, 0.2" />
	<connect from="texcoord camera" to="bricktex vector" />
	<connect from="bricktex color" to="bg color" />
	<connect from="bg background" to="output surface" />
Complete support for Gamma node.

Resolves #20
If world shader doesn't use nodes this could lead errors using None.
Add support for Layer weight node.

Resolves #21
Add support for Voronoi Texture.

Resolves #19
Remove dead code.
Fresnel and Facing sockets weren't added properly
Add support for Fresnel node, Mix Color node, Mix Shader node.
Add smooth flag to set_tris API call.