vo_opengl: add gamma-auto option

This automatically sets the gamma option depending on lighting conditions
measured from the computer's ambient light sensor.

sRGB – arguably the “sibling” to BT.709 for still images – has a reference
viewing environment defined in its specification (IEC 61966-2-1:1999, see
http://www.color.org/chardata/rgb/srgb.xalter). According to this data, the
assumed ambient illuminance is 64 lux. This is the illuminance where the gamma
that results from ICC color management is correct.

On the other hand, BT.1886 formalizes that the gamma level for dim environments
to be 2.40, and Apple resources (WWDC12: 2012 Session 523: Best practices for
color management) define the BT.1886 dim at 16 lux.

So the logic we apply is:

  * >= 64lux -> 1.961 gamma
  * =< 16lux -> 2.400 gamma
  * 16lux < x < 64lux -> logaritmic rescale of lux to gamma. The human
    perception of illuminance roughly follows a logaritmic scale of lux [1].

[1]: https://msdn.microsoft.com/en-us/library/windows/desktop/dd319008%28v=vs.85%29.aspx

DOCS/man/vo.rst

@@ -543,6 +543,15 @@ Available video output drivers are:
         0.8
             Pitch black room
 
+    ``gamma-auto``
+        Automatically corrects the gamma value depending on ambient lighting
+        conditions (adding a gamma boost for dark rooms).
+
+        With ambient illuminance of 64lux, mpv will pick the 1.0 gamma value
+        (no boost), and slightly increase the boost up until 0.8 for 16lux.
+
+        NOTE: Only implemented on OS X.
+
     ``icc-profile=<file>``
         Load an ICC profile and use it to transform linear RGB to screen output.
         Needs LittleCMS 2 support compiled in. This option overrides the ``srgb``