vo_opengl: disable extended downscaling by default

To simplify implementation, the same filter kernel was used for both
directions, even when the scaling factors were different. It turns
out that people actually did this, and that the resulting rendering
errors were rather visible. Disable this feature by default, as
fixing it would require structural changes, and it's a useless anyway.

DOCS/man/en/vo.rst

@@ -456,7 +456,7 @@ opengl-hq
 
     This is equivalent to:
 
-    | --vo=opengl:lscale=lanczos2:fancy-downscaling:dither-depth=0:pbo:fbo-format=rgb16
+    | --vo=opengl:lscale=lanczos2:dither-depth=0:pbo:fbo-format=rgb16
 
     Note that some cheaper LCDs do dithering that gravely interferes with
     vo_opengl's dithering. Disabling dithering with ``dither-depth=-1`` helps.

libvo/vo_opengl.c

@@ -2148,7 +2148,6 @@ static int preinit(struct vo *vo, const char *arg)
         .dither_depth = hq ? 0 : -1,
         .fbo_format = hq ? GL_RGB16 : GL_RGB,
         .use_scale_sep = 1,
-        .use_fancy_downscaling = hq,
         .scalers = {
             { .index = 0, .name = hq ? "lanczos2" : "bilinear" },
             { .index = 1, .name = "bilinear" },