A delta that can be applied to a VirtualSpace
to change its state. Examples of deltas are
a glyph position or color change, a rectangle creation,
a glyph deletion, ...
Deltas are sent over the wire and therefore should be
Important note: this interface is called 'Delta' because
a Delta enables us to move from one VirtualSpace state to
It does *not* imply a glyph- or camera-level delta, i.e.
GlyphPosDelta will probably store *absolute* coordinates and
internally use moveTo and not move. That said, it should not
make any difference for users. It is simply a design choice.