Add Canvas and OpenGLCanvas widgets

[rdbende]

TODO:

• Features of the traditional canvas widget:
  https://www.tcl.tk/man/tcl8.6/TkCmd/canvas.html

• Features of the TkPath canvas widget
  https://github.com/avarga/tkpath/blob/master/doc/readme.adoc

• OpenGL rendering

  • OpenGlCanvas class
  • moderngl or pyopengl?
    Moderngl is faster and more pythonic, but then we have to implement every platform specific stuff manually (context creation, makecurrent and swapbuffer)

• Implement new dynamic theming engine using tkp::surface

TODO in TkPath:

• Add to libtukaan. Currently I have only a local .so
• Some commands cause segfaults randomly. I've put some TODO comments to them
• Finish partially or not implemented features

Done (at least partially):

These things need to be tested and possibly bugfixed

• Canvas items
  • Line (connects two points)
  • Vector (has a length and a angle)
    See comment about these two in items.py
  • Polygonal line
  • Rectangle
  • Square (rectangle subclass)
  • Ellipse
  • Circle (ellipse subclass)
  • Polygon
  • Text
  • Image
  • Path
  • Item group
• Utility classes
  • Arrowheads
  • Stroke patterns
  • Transformations
  • Pen (to draw object stroke)
  • Brush (to draw object fill)

• TkPath works on Linux

[rdbende]

Maybe revert this, and use a global tkfont instance to convert points to pixels in get_font_options_for_text_item

[rdbende]

This is super unintuitive, and makes no sense.

Unfortunately we can't make it totally independent from the camvas wodget, because that's not how TkPath works, but it shouldn't be a "method".

Plus another thing came into my mind, that's actually a serious problem. There are random helper objects that are mutable and there are others that are immutable. This is because the objects that are mutable are actual Tk stuff, and others are just simple Python objects, e.g dataclasses, which can't notify the widget is an attribute is changed. This is NOT good. Something must be done.

Mutable:

• Font
• gradients
• Control variables

Immutable

• Color
• Brush and Pen
• ArrowHead

[sumeshir26]

What do mutable and unmutable objects mean?
As far as I know, a object is a instance of a class right?

[rdbende]

Mutable and immutable are just what they sound like. A mutable object can be mutated, i.e. modified, while an immutable cannot.

# Mutable
foo = [1, 2, 3]
foo[2] = 4  # Works, because lists are mutable

# Immutable
bar = (1, 2, 3)
bar[2] = 4  # Throws an error obviously

In this case for example the font object is mutable. So if you change its font family or size after you assigned it to a widget, the widget will pick up the changes. While when you modify parameters of a color object that you've previously assigned to a widget, nothing will happen.

[sumeshir26]

Interesting.

[rdbende]

Honestly I'm quite dissatisfied with this.

[rdbende]

For example, this might seem a sane thing:

line = Line(canvas, pen=my_fancy_pen)
line.draw(0, 0, 20, 20)

especially considering that widgets are created and placed in the same way basically.

But on the other hand it's quite inconvenient and unintuitive to do.
Imo it's much cleaner to do it like this:

line = canvas.create_line_between(Point(0, 0), Point(20, 20), pen=my_fancy_pen)