[Rd] persp - incorrect shading of triangular facets (PR#4731)

[ihaka at stat.auckland.ac.nz]

Full_Name: Ross Ihaka
Version: all versions containing persp
OS: linux
Submission from: (NULL) (130.216.191.167)


When is persp is invoked on a matrix of heights which contains missing values,
the surface which is drawn will contain triangular facets.  The shading
computations for these facets are incorrect and always produce black facets.
(I plan to fix this - the report is just in case I forget or get hit by a bus).