Dodge and Burn brush modes

[rezaularif]

Adds Dodge and Burn as Brush modes: Dodge lightens and Burn darkens the pixels under the stroke. Closes #169.

What's added

• Two new Brush modes beside Paint and Erase.
• Range (Shadows, Midtones or Highlights) and Exposure (0–100%) in the Brush bar.
• Overlapping dabs build up to the stroke's strength and never past it. Alpha is left alone, so transparent pixels stay transparent.
• As [Feature] Add Dodge and Burn Brush Modes #169 asks: a mask as the target is refused with a message, and a stroke that changes nothing, or one at 0% exposure, leaves no undo step.

How to use

• Pick the Brush (B) and choose Dodge or Burn in the mode picker. Tab steps through Paint, Erase, Dodge and Burn.
• Set Range and Exposure in the bar, then paint. Size, hardness, opacity, smoothing, the selection and Shift-click straight lines work as they do for Paint.
• Each stroke is one undo step, named Dodge or Burn.

Tests

• DodgeBurnTests. The full suite passes, and the project format is unchanged.
• The same stroke across a 4000 × 4000 photo draws each frame in about the same time as Paint: medians of 0.3 ms at 40 px and about 2 ms at 800 px.