wined3d: Make the surface parameter to surface_convert_depth_to_float() const.

dlls/wined3d/surface.c

@@ -1212,7 +1212,7 @@ static BOOL fbo_blit_supported(const struct wined3d_gl_info *gl_info, enum wined
     return TRUE;
 }
 
-static BOOL surface_convert_depth_to_float(struct wined3d_surface *surface, DWORD depth, float *float_depth)
+static BOOL surface_convert_depth_to_float(const struct wined3d_surface *surface, DWORD depth, float *float_depth)
 {
     const struct wined3d_format *format = surface->resource.format;