UI improvements and bug fixes

[mwomick]

Description

Major changes:

• [Feature] Pre-render the canvas so we don't have to redraw the canvas on every draw update.
• [Bug fix] Closing polygon using 'Enter' now clears the canvas before redrawing.
• [Feature] Do not accumulate darkness of overlapping shapes. Instead, fill all polygons at the same time and blend colors once for a uniform fill.
• [Bug fix] Immediately refresh the canvas on undo. Previously, one had to move the mouse to update the canvas.

Other changes:

• Allow user to press upper or lower case letters to change draw mode (i.e. 'l'/'L' and 'p'/'P'). This reduces confusion because the documentation currently says press 'L' or 'P' but it means 'l' or 'p'. Now either are acceptable.
• Some minimal refactoring
• Now colors cycle periodically instead of randomly. The old method permitted some double coloring after all the colors in color_choices had been exhausted.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How has this change been tested, please provide a testcase or example of how you tested the change?

Tested manually in Safari and Chrome.

Any specific deployment considerations

Some documentation might need to be updated to reflect the change in the keys used to switch draw modes.

[mwomick]

@capjamesg I made a pull request this weekend that solves most of the remaining issues I identified and also added some minor features and optimizations. The main issue remaining is saving the canvas, but I don't want to prioritize that now since it's a tricky problem and doesn't seem to be a frequently used feature.

[capjamesg]

This looks great! I will merge now.

One idea for the future: It would be amazing if Ctrl + Z (undo) let you undo your last polygon.