Add circle and square borderless shaders

[melgrove]

Benchmarked to be faster than default point shader implementation because it doesn't call mix() to make a border. Used by setting:

• fragmentShaderSource: L.glify.shader.fragment.dotBorderless
• fragmentShaderSource: L.glify.shader.fragment.squareBorderless

I've only tested this for the points() method. Not sure what it would do for the others.

[melgrove]

Closes #111