Add support for GL_POINTS and gl_PointSize

[h-isthebestletter]

Simple pull request to add support for drawing Points as a primitive as well as support for using gl_PointSize in the vertex shader. Sorry if I missed anything, I don't know much about OpenGL. However I've used it in my own toy project and it works (not sure how much of an assurance that is)

Now you should be able to make a pipeline that has gl_PointSize support by doing

let pipeline = gfx.create_pipeline()
    .from(&VERTEX, &FRAGMENT)
    .with_vertex_info(&vertex_info)
    .with_point_size_available(true) // note
    .build()
    .unwrap();

And you should be able to draw points by doing

renderer.set_primitive(DrawPrimitive::Points);