PyDriver support for all RNA property types

Support for driver variables that don't resolve to numbers, eg:
objects, bones, curves... etc.

Without this, Python expressions to access this data needed to use an absolute path from `bpy.data`,
however this is inconvenient, breaks easily (based on naming) and wouldn't set the dependencies correctly.

source/blender/python/intern/bpy_rna_driver.c

@@ -0,0 +1,79 @@
+/*
+ * ***** BEGIN GPL LICENSE BLOCK *****
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2
+ * of the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ *
+ * ***** END GPL LICENSE BLOCK *****
+ */
+
+/** \file blender/python/intern/bpy_rna_driver.c
+ *  \ingroup pythonintern
+ *
+ * This file defines utility functions that use the RNA API, from PyDrivers.
+ */
+
+#include <Python.h>
+
+#include "MEM_guardedalloc.h"
+
+#include "BLI_utildefines.h"
+
+#include "BKE_fcurve.h"
+
+#include "RNA_access.h"
+
+#include "bpy_rna.h"
+
+#include "bpy_rna_driver.h"  /* own include */
+
+
+/**
+ * A version of #driver_get_variable_value which returns a PyObject.
+ */
+PyObject *pyrna_driver_get_variable_value(
+        struct ChannelDriver *driver, struct DriverTarget *dtar)
+{
+	PyObject *driver_arg = NULL;
+	PointerRNA ptr;
+	PropertyRNA *prop = NULL;
+	int index;
+
+	if (driver_get_variable_property(driver, dtar, &ptr, &prop, &index)) {
+		if (prop) {
+			if (index != -1) {
+				if (index < RNA_property_array_length(&ptr, prop) && index >= 0) {
+					/* object, property & index */
+					driver_arg = pyrna_array_index(&ptr, prop, index);
+				}
+				else {
+					/* out of range, pass */
+				}
+			}
+			else {
+				/* object & property */
+				driver_arg = pyrna_prop_to_py(&ptr, prop);
+			}
+		}
+		else {
+			/* object only */
+			driver_arg = pyrna_struct_CreatePyObject(&ptr);
+		}
+	}
+	else {
+		/* can't resolve path, pass */
+	}
+
+	return driver_arg;
+}