Pen pressure for the Brush's size and opacity

[rezaularif]

Adds pen pressure for the Brush's size and opacity, on a graphics tablet.

What's added

• Two buttons beside the Brush's Opacity, as in Photoshop: pressure sets the size, or pressure sets the opacity. Both are off by default.
• Size: a light touch paints a thinner line, down to one pixel.
• Opacity: a light touch paints fainter, up to the brush's Opacity, and a stroke never gets more opaque than its firmest press.
• Pressure is carried to both the GPU brush and the software fallback, and eases smoothly along the stroke.
• Only tablet events carry pressure. A mouse, or a Force Touch trackpad, paints at full pressure, so those strokes are unchanged.

How to use

• Pick the Brush (B), in Paint or Erase, and turn on either or both pressure buttons next to Opacity.
• Paint with a tablet pen. Press harder for a wider or stronger line, lighter for a thinner or fainter one.

Tests

• PenPressureTests, using simulated tablet events. The full suite passes, and the project format is unchanged.
• It hasn't been tried on a physical tablet yet.
• Frame times with a pen match a mouse on a 4000 × 4000 photo.

If Dodge and Burn (#193) is merged first, BrushSettings and the README have a small conflict. Keep both sides.