Brainstorming/Brushmodifier

Ways to store/restore temporary brushes

 * devbrush (stylus / eraser / 2nd stylus)
 * brush history (recently used brushes)
 * context (save brush with Ctrl-4, restore with 4)
 * strokemap (pick stroke from canvas)
 * autosave (restore last radius etc. when using the brush selector)

Eraser Mode

 * Eraser mode does not survive brush selection.
 * If eraser mode was explicitly set by the user, it is (supposed to be) auto-canceled when selecting a color.
 * Eraser mode remembers the radius change relative to normal mode (eg. small ink brush, bigger ink eraser)
 * Eraser mode is more convenient than selecting an eraser brush because you can toggle it with a single key. (But independent from that, the possibility to erase with any brush shape is also appreciated by users.)

Lock Alpha

 * Lock Alpha is currently a persistent mode that survives brush selection.
 * Technically it can be combined with Eraser mode, but that does not do anything useful.
 * Most applications make it a setting of the layer. If we want to support this via brushmodifier, then the change applied by brushmodifier definitively should not be stored anywhere.
 * Brush presets may have "Lock Alpha" enabled, but there probably is no good usecase except testing.

Brush Blend Modes

 * Brush presets may have blend modes enabled (eg. the brush selector could offer desaturate, brigthen and darken brushes)
 * It may make sense to combine them with Lock Alpha (work on current layer / work on composited image).
 * A brush blend mode can also be enabled for all brushes, in which case it is a persistent mode that survives brush selection.
 * Maybe it should be remembered per device(?)