Blender exporter: remove colorAmbient

ArrowHelper: fix undefined reference when using zero-length tails

Earcut text geometry

Commit 9e2103f9 added support for zero-length tails by conditionally
creating the line object. However it kept using that object
unconditionally in some places (such as setLength), which results in
undefined references if an ArrowHelper is initially created with a
zero-size tail and then made longer by calling setLength().

This patch fixes this problem by always creating the tail object and
limiting its minimum length to 0. This also simplifies the code a bit.

A simple example that illustrates selective drawing of BufferGeometry

Fixed vertex shader draw order and replaced x_x variables/functions with camel case versions as per standard.

Moved SpriteCanvasMaterial.html to examples (docs), Fixed src link