Improve quality of pen

[towerofnix]

Expected Behavior

Pen lines are smooth, similar to vector lines drawn in the paint editor.

Note this is a suggestion (from this post), not a compatibility report, because 2.0 also has a pixellated pen.

Actual Behavior

The bitmap quality of the pen is quite low:

Steps to Reproduce

Draw something with the pen. Here's the project that I made for that screenshot: https://llk.github.io/scratch-gui/develop/#291041723

Operating System and Browser

Linux (Debian), Firefox 66.0b12.

[thisandagain]

/cc @cwillisf