LineMaterial: Refactor GLSL.

[Mugen87]

Related issue: https://discourse.threejs.org/t/warning-output-of-vertex-shader-worldstart-not-read-by-fragment-shader/31125

Description

Depending on how LineMaterial is configured, the following warnings can occur:

THREE.WebGLProgram: Program Info Log: WARNING: Output of vertex shader 'worldStart' not read by fragment shader
THREE.WebGLProgram: Program Info Log: WARNING: Output of vertex shader 'vUv' not read by fragment shader

The PR fixes these warnings by introducing some #ifdef statements so varyings are only defined if they are actually required.

[WestLangley]

/ping @gkjohnson

[gkjohnson]

Awesome -- thank you!

[mrdoob]

Thanks!

[bernhard-42]

@Mugen87 Thanks a lot, this was really fast - it's already part of r134!