Add DEPTH fragment built-in

tutorials/shading/shading_language.rst

@@ -872,6 +872,8 @@ Fragment Built-Ins
 +----------------------------------+--------------------------------------------------------------------------------------------------+
 | sampler2D **DEPTH_TEXTURE**      | Built-in Texture for reading depth from the screen. Must convert to linear using INV_PROJECTION. |
 +----------------------------------+--------------------------------------------------------------------------------------------------+
+| out float **DEPTH**              | Custom depth value (0..1).                                                                       |
++----------------------------------+--------------------------------------------------------------------------------------------------+
 | out vec2 **SCREEN_UV**           | Screen UV coordinate for current pixel.                                                          |
 +----------------------------------+--------------------------------------------------------------------------------------------------+
 | in vec2 **POINT_COORD**          | Point Coord for drawing points with POINT_SIZE.                                                  |