bindings/python: Add priv_drop_user and priv_drop_group

This gives Python users control over which user/group the process is
dropped into.

bindings/python/rgbmatrix/core.pxd

@@ -19,6 +19,8 @@ cdef class RGBMatrixOptions:
     cdef bytes __py_encoded_led_rgb_sequence
     cdef bytes __py_encoded_pixel_mapper_config
     cdef bytes __py_encoded_panel_type
+    cdef bytes __py_encoded_drop_priv_user
+    cdef bytes __py_encoded_drop_priv_group
 
 # Local Variables:
 # mode: python

bindings/python/rgbmatrix/core.pyx

@@ -192,6 +192,18 @@ cdef class RGBMatrixOptions:
         def __get__(self): return self.__runtime_options.drop_privileges
         def __set__(self, uint8_t value): self.__runtime_options.drop_privileges = value
 
+    property drop_priv_user:
+        def __get__(self): return self.__runtime_options.drop_priv_user
+        def __set__(self, value):
+            self.__py_encoded_drop_priv_user = value.encode('utf-8')
+            self.__runtime_options.drop_priv_user = self.__py_encoded_drop_priv_user
+
+    property drop_priv_group:
+        def __get__(self): return self.__runtime_options.drop_priv_group
+        def __set__(self, value):
+            self.__py_encoded_drop_priv_group = value.encode('utf-8')
+            self.__runtime_options.drop_priv_group = self.__py_encoded_drop_priv_group
+
 cdef class RGBMatrix(Canvas):
     def __cinit__(self, int rows = 0, int chains = 0, int parallel = 0,
         RGBMatrixOptions options = None):

bindings/python/rgbmatrix/cppinc.pxd

@@ -35,6 +35,8 @@ cdef extern from "led-matrix.h" namespace "rgb_matrix":
       int gpio_slowdown
       int daemon
       int drop_privileges
+      const char *drop_priv_user
+      const char *drop_priv_group
 
 
     RGBMatrix *CreateMatrixFromOptions(Options &options, RuntimeOptions runtime_options)