sagemath#34995 (trac sagemath#23712): support tachyon >= 0.99.2

In tachyon 0.99.2 the keyword `focallength` was changed to `focaldist`.
To support it, when running on version >= 0.99.2 we "patch" the model as
constructed by class `sage.plot.plot3d.tachyon.Tachyon`.

In the future (possibly when tachyon in sage gets upgraded), all the
focallength occurences in sage.plot.plot3d.tachyon can be replaced by
focaldist for consistency with new tachyon, and the logic here can be
reversed (i.e. patch the model when self.version() < '0.99.2') or just
drop support for old versions.

src/sage/interfaces/tachyon.py

@@ -683,12 +683,14 @@
 #*****************************************************************************
 
 import os
+import re
 
 from sage.cpython.string import bytes_to_str
 from sage.misc.pager import pager
 from sage.misc.superseded import deprecation
 from sage.misc.temporary_file import tmp_filename
 from sage.structure.sage_object import SageObject
+from sage.misc.cachefunc import cached_method
 
 
 class TachyonRT(SageObject):
@@ -799,6 +801,11 @@ def __call__(self, model, outfile='sage.png', verbose=1, extra_opts=''):
             Parser failed due to an input file syntax error.
             Aborting render.
         """
+        if self.version() >= '0.99.2':
+            # this keyword was changed in 0.99.2
+            model = model.replace(
+                    "              focallength ",
+                    "              focaldist ")
         modelfile = tmp_filename(ext='.dat')
         with open(modelfile, 'w') as file:
             file.write(model)
@@ -851,6 +858,22 @@ def usage(self, use_pager=True):
         else:
             print(r)
 
+    @cached_method
+    def version(self):
+        """
+        Returns the version of the Tachyon raytracer being used.
+
+        TESTS::
+
+            sage: tachyon_rt.version()  # not tested
+            0.98.9
+            sage: tachyon_rt.version() >= '0.98.9'
+            True
+        """
+        with os.popen('tachyon') as f:
+            r = f.read()
+        return re.search(r"Version ([\d.]*)", r)[1]
+
     def help(self, use_pager=True):
         """
         Deprecated: type 'sage.interfaces.tachyon?' for help