vo_opengl: implement HDR (SMPTE ST2084)

Currently, this relies on the user manually entering their display
brightness (since we have no way to detect this at runtime or from ICC
metadata). The default value of 250 was picked by looking at ~10 reviews
on tftcentral.co.uk and realizing they all come with around 250 cd/m^2
out of the box. (In addition, ITU-R Rec. BT.2022 supports this)

Since there is no metadata in FFmpeg to indicate usage of this TRC, the
only way to actually play HDR content currently is to set
``--vf=format=gamma=st2084``. (It could be guessed based on SEI, but
this is not implemented yet)

Incidentally, since SEI is ignored, it's currently assumed that all
content is scaled to 10,000 cd/m^2 (and hard-clipped where out of
range). I don't see this assumption changing much, though.

As an unfortunate consequence of the fact that we don't know the display
brightness, mixed with the fact that LittleCMS' parametric tone curves
are not flexible enough to support PQ, we have to build the 3DLUT
against gamma 2.2 if it's used. This might be a good thing, though,
consdering the PQ source space is probably not fantastic for
interpolation either way.

Partially addresses #2572.

DOCS/man/vo.rst

@@ -1052,6 +1052,14 @@ Available video output drivers are:
             Pure power curve (gamma 2.8), also used for BT.470-BG
         prophoto
             ProPhoto RGB (ROMM)
+        st2084
+            SMPTE ST2084 (HDR) curve, PQ OETF
+
+    ``target-brightness=<1..100000>``
+        Specifies the display's approximate brightness in cd/m^2. When playing
+        HDR content, video colors will be scaled and clipped to this
+        brightness. The default of 250 cd/m^2 corresponds to a typical consumer
+        display.
 
     ``icc-profile=<file>``
         Load an ICC profile and use it to transform video RGB to screen output.