[gui] Add wareframe mode for mesh & mesh_instance, add slider_int for Window.GUI.

[Morcki]

Note:
New features:

1. Support three display modes: Fill Mode (fill faces), Line Mode (Wareframe), Point Mode (rendering as points).
2. Add a new widget slider_int in window.GUI, which supports slide int type value from minimum to maximum.
3. Add an arg named instance_count to control the count of mesh instances.

Usage:

window = ti.ui.Window("Display Mesh", (1024, 1024), vsync=True)
canvas = window.get_canvas()
scene = ti.ui.Scene()
camera = ti.ui.make_camera()

# slider_int usage
some_int_type_value = 0
def show_options():
    global some_int_type_value

    window.GUI.begin("Display Panel", 0.05, 0.1, 0.2, 0.15)
    display_mode = window.GUI.slider_int("Value Range", some_int_type_value, 0, 5)
    window.GUI.end()

while window.running:
    
    ...
    # if to show wareframe 
    scene.mesh_instance(vertices, indices, instance_count = 100 , show_wareframe = True)
    
    canvas.scene(scene)
    show_options()
    window.show()

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[ailzhang]

Thanks! Had a small suggestion w.r.t using enums instead of raw ints

[YuCrazing]

LGTM!

[ailzhang]

Looks a lot better! I had a few more nits inline and then this PR is good to go

[PENGUINLIONG]

again, i still suggest you only to differentiate filled and wireframe draws, it would be a simple boolean instead of all the struggles with DisplayMode. I cannot come up with a case that you wanna debug meshes without the connectivity information between vertices and you still don't think scene.particles is a first choice. the structure of input field or ndarray ought to be the same either way.

[Morcki]

again, i still suggest you only to differentiate filled and wireframe draws, it would be a simple boolean instead of all the struggles with DisplayMode. I cannot come up with a case that you wanna debug meshes without the connectivity information between vertices and you still don't think scene.particles is a first choice. the structure of input field or ndarray ought to be the same either way.

I agree with that, I will pass a bool value to enable users to decide to show WareFrame mode or not to.

[ailzhang]

LGTM! Thanks!