feat: render smear as quad

[sphamba]

Changes

• remove draw_line and used functions
• add draw_quad
• render smear as quad instead of two linked rectangles

Related GitHub issues and pull requests

• necessary for Feature Request: Stretch the cursor to the tab width #7 and Is vertical bar supported or only block shape? #33
• fix Support arbitrary cursor width #34

[sphamba]

@folke Hey! I completely rewrote the way the smear is rendered in order to address other issues. I wonder how it runs for you and if you could do some profiling :)

[folke]

Looks great!! And performance fine as well.
I actually released the snacks profiler yesterday, so you can now use it too if you want