vo_opengl: use textureGatherOffset for polar filters

This is more efficient on my machine (nvidia), but only when applied to
groups of exactly 4 texels. So we switch to the more efficient
textureGather for groups of 4. Some notes:

- textureGatherOffset seems to be faster than textureGather by a
  non-negligible amount, but for some reason, textureOffset is still
  slower than a straight-up texture
- textureGather* requires GLSL 400; and at least on nvidia, this
  requires actually allocating a GL 4.0 context.
- the code in opengl/common.c that clamped the GLSL version to 330 is
  deprecated, because the old user shader style has been removed
  completely in the meantime
- To combat the growing complexity of the polar sampling code, we drop
  the antiringing functionality from EWA shaders completely, since it
  never really worked well for EWA to begin with. (Horrific artifacting)

1 files changed, 1 insertions, 1 deletions

diff --git a/DOCS/man/options.rst b/DOCS/man/options.rst
index 94a153408a..896a6f3dad 100644
--- a/DOCS/man/options.rst
+++ b/DOCS/man/options.rst
@@ -4034,7 +4034,7 @@ The following video options are currently all specific to ``--vo=opengl`` and
     0.0 and 1.0. The default value of 0.0 disables antiringing entirely.
 
     Note that this doesn't affect the special filters ``bilinear`` and
-    ``bicubic_fast``.
+    ``bicubic_fast``, nor does it affect any polar (EWA) scalers.
 
 ``--scale-window=<window>``, ``--cscale-window=<window>``, ``--dscale-window=<window>``, ``--tscale-window=<window>``
     (Advanced users only) Choose a custom windowing function for the kernel.