Update Public APIs to support different renderers

[arihantparsoya]

Currently all the APIs use PShape only for rendering shapes as vispy is the only renderer being used. To add support for new renderers, we want to update the existing API functions to call that specific renderer. Pseudo code:

def shapeXYZ(): # some rendering function
    renderer.shapeXYZ() # the function corresponding to the renderer will be called.

[arihantparsoya]

Few more things needs to be done after #288

1. Implement begin_shape() API inside the Vispy2DRenderer

[arihantparsoya]

2. Test if the Font and Image APIs are working as expected with the new library structure.

[tushar5526]

It skipped my mind to refactor image.py as it has a dependency on Texture2D from vispy.gloo. Image APIs are working fine as of now, as there is only one renderer.
PImage has to be updated to support multiple renderers. I will make a PR soon.

[tushar5526]

Few more things needs to be done after #288

1. Implement begin_shape() API inside the Vispy2DRenderer

@parsoyaarihant Can you tell a bit more about this? I am interested in doing it.

[arihantparsoya]

begin_shape() API currently uses PShape: https://github.com/p5py/p5/blob/master/p5/core/vertex.py#L335

We need to move this code into the VispyRenderer.