v0.111.6

Minor changes

• Added a panning example to the help page for setUserCallbacks().
• Replaced all calls to sprintf from C/C++ code with calls to
  snprintf.
• surface3d and rgl.surface are now more flexible,
  allowing any of the 3 coordinates to be a vector or matrix
  as long as at least one is a matrix.
• material3d can now specify an id to query properties
  for individual objects.
• Since rgl.material is soon to be deprecated and no
  longer contain the list of material types in its
  argument list, rgl.material.names and rgl.material.readonly have been
  added.
• Similarly, rgl.par3d.names and rgl.par3d.readonly
  contain lists of properties that may be set or queried
  in par3d().
• Made some examples conditional on interactive use
  to save time on CRAN.

Bug fixes

• Default mouse modes used when a window is opened by an rgl.*
  call (which is not recommended!) now match
  the defaults in rgl::r3dDefaults.
• Missing values could cause surface3d() to segfault.
• The C source code for gl2psGetFileFormat missed declaring
  a prototype.