Fixed -Wmissing-variable-declarations warnings in headers

Fixed by adding 'const' to several items.

As this changes headers, it might have an impact on
clients, but I don't think so...

Change-Id: Ib975e219e6e30e83ec2fc25feeae389b4f10507f
Fixed -Wmissing-variable-declarations warnings in .cxx

Fixed by adding 'static' and/or 'const' to several
variables that had neither.

Removed one obsolete comment too.

Change-Id: I8775c9a1fa8d9cc75b28619216bbe7f901117a0f
Remove the MathTextActor classes.

* Converted tests to use regular TextActors.
* Fixed the MathText path generator to rotate paths if needed.
* Update the GL2PS text export code to check for mathtext
  strings, since mathtext/freetype are both handled by the same
  actor.
* Made vtkTextRenderer::DetectBackend public so we can peek at
  the backend from the export code.

Change-Id: I52ba3c1e916c70dd80b67d59b611db1becb0846c
Merge topic 'Wmissing-variable-declarations' into master

648903ed Fixed -Wmissing-variable-declarations warnings in .cxx
7f8ca8e8 Fixed -Wmissing-variable-declarations warnings in headers
Merge topic 'remove-mathtextactors' into master

d978780f Remove the MathTextActor classes.
Removed dead code found by compiler warning

After fixing one warning about private globals
not being marked static (by marking them static)
another warning appeared saying that they are in fact
not used at all.  Just removed them.

Change-Id: Iead6576f9106ab3207f0b9452f7a6951379d8f3d
Merge topic 'fixed-defined-but-not-used' into master

8ff3310f Removed dead code found by compiler warning