Fix #888: Add ly:stencil-scale.

Fix #888: Add ly:stencil-scale.

* input/regression/stencil-scale.ly

  new regtest for testing ly:stencil-scale

* lily/stencil-interpret.cc (interpret_stencil_expression):

  interpret new stencil command `scale-stencil', unscaling offset of body to
  compensate for change in output scale

* lily/stencil-scheme.cc (ly_stencil_scale):

  export new function `ly:stencil-scale'

* lily/stencil.cc, include/stencil.hh (scale):

  new method for scaling stencils

* scm/define-markup-commands.scm (scale):

  new markup command

* scm/define-stencil-commands.scm

  (ly:all-stencil-commands): add stencil commands for setting and resetting
  scale (`setscale', `resetscale')

  (ly:all-output-backend-commands): add `scale-stencil' command

* scm/output-ps.scm, output-svg.scm (setscale, resetscale):

  implement stencil outputters for scaling