vo_opengl: rename custom shader entrypoint from sample to sample_pixel

"sample" is a reserved identifier at least in GLES ES. Suggestions for a
better name than "sample_pixel" are still welcome.

Fixes #2733.

DOCS/interface-changes.rst

@@ -25,6 +25,8 @@ Interface changes
     - add --audio-normalize-downmix
     - change the default downmix behavior (--audio-normalize-downmix=yes to get
       the old default)
+    - VO opengl custom shaders must now use "sample_pixel" as function name,
+      instead of "sample"
  --- mpv 0.15.0 ---
     - change "yadif" video filter defaults
  --- mpv 0.14.0 ---