Add depth tinting to cave rendermode.

src/rendermodes.h

@@ -123,12 +123,16 @@ extern RenderModeInterface rendermode_spawn;
 typedef struct {
     /* render blocks with lighting mode */
     RenderModeNormal parent;
+
     /* data used to know where the surface is */
     PyObject *skylight;
     PyObject *left_skylight;
     PyObject *right_skylight;
     PyObject *up_left_skylight;
     PyObject *up_right_skylight;
+
+    /* colors used for tinting */
+    PyObject *depth_colors;
     
 } RenderModeCave;
 extern RenderModeInterface rendermode_cave;