Add pen presure to brush

[Giwayume]

I had to change more than expected,

• Added pointerdown and pointermove events to take a pressure reading
• Existing code had a bug where it generated a "point" at the start of every line after the 1st line, so I forced a "null" data point after each mouseup so you can truly distinguish points from lines. This was important to fix because the pen pressure can cause lines to be much thinner than points, so it's noticeable.

[Giwayume]

#179

[viliusle]

I updated brush tool, now it supports multi touch, so user can draw with many fingers at once.

Can you please check if pressure is not broken?

[Giwayume]

pressure still works fine