'disk' emitter, radiusSpread, angles

[stemkoski]

• Included disk-shaped emitter
• now able to randomize radius size for sphere/disk emitter
• particles can have an angle, angle can be randomized or aligned to velocity direction
• fixed _resetParticle velocity direction issue
• TODO: incorporate angleVelocity for rotating particles

[stemkoski]

The pull requests #8 and #9 go hand-in-hand, one is for the particle emitter and the other for the group. Try testing it out with the following test case:

// Create a particle group to add the emitter
this.particleGroup = new ShaderParticleGroup(
{
    texture: THREE.ImageUtils.loadTexture('images/laserWhiteSmall.png'),
    maxAge: 1,
    colorize: 1,
    blending: THREE.NormalBlending,
});

// Create a single emitter
var particleEmitter = new ShaderParticleEmitter(
{
    type: 'disk',
    position: new THREE.Vector3(0, 100, 0),
    radius: 20,
    radiusSpread: 40,
    speed: 50,
    speedSpread: 10,
    colorStart: new THREE.Color('yellow'),
    colorEnd: new THREE.Color('red'),
    opacityStart: 1,
    opacityMiddle: 1,
    opacityEnd: 0,
    size: 20,
    sizeEnd: 20,
    angle: 0,
    // angleSpread: 2 * Math.PI, // use for random direction
    angleAlignVelocity : true,
    particlesPerSecond: 1000,
    emitterDuration: 5 // null = infinite duration
});

// Add the emitter to the group.
particleGroup.addEmitter( particleEmitter );

// Add the particle group to the scene so it can be drawn.
scene.add( particleGroup.mesh );

With the image included below:

and you should get something resembling the following: