Drivers: add lerp and clamp functions to namespace.

Implementation of lerp without a function requires repeating one of
the arguments, which is not ideal. To avoid that, add a new function
to the driver namespace. In addition, provide a function for clamping
between 0 and 1 to support easy clamped lerp, and a smoothstep function
from GLSL that is somewhat related.

The function implementations are added to a new bl_math module.
As an aside, add the round function and two-argument log to the
pylike expression subset.

Differential Revision: https://developer.blender.org/D8205

source/blender/python/intern/bpy_driver.c

@@ -114,6 +114,19 @@ int bpy_pydriver_create_dict(void)
     Py_DECREF(mod);
   }
 
+  /* Add math utility functions. */
+  mod = PyImport_ImportModuleLevel("bl_math", NULL, NULL, NULL, 0);
+  if (mod) {
+    static const char *names[] = {"clamp", "lerp", "smoothstep", NULL};
+
+    for (const char **pname = names; *pname; ++pname) {
+      PyObject *func = PyDict_GetItemString(PyModule_GetDict(mod), *pname);
+      PyDict_SetItemString(bpy_pydriver_Dict, *pname, func);
+    }
+
+    Py_DECREF(mod);
+  }
+
 #ifdef USE_BYTECODE_WHITELIST
   /* setup the whitelist */
   {
@@ -133,6 +146,10 @@ int bpy_pydriver_create_dict(void)
         "bool",
         "float",
         "int",
+        /* bl_math */
+        "clamp",
+        "lerp",
+        "smoothstep",
 
         NULL,
     };