Blender Git Loki

Fix T81334: Python view-port drawing depth-test regression

Since 216d78687d2b9468b05fb598d1cef0b8424a40d2 the depth function
(glDepthFunc) was left in an undefined state for drawing callbacks that
use the `bgl` module.

This meant enabling depth-test from Python's bgl module also needed
to set the depth function (which previously wasn't necessary).

Set the depth function as part of GPU_bgl_start