vo_opengl: slightly improve ewa_lanczos windowing

The original filter window was design for a radius based on the true
zero, but we always cut it off at our selection of radius either way (by
necessity, due to the square matrix we sample from).

This window is tweaked from the original (true radius) to our actual
cut-off radius, and hence improves the result in a few edge cases. The
main win is the reduction of code complexity, since we no longer need to
know what the true radius actually is.

video/out/filter_kernels.c

@@ -307,43 +307,14 @@ static double ginseng(kernel *k, double x)
 static double ewa_lanczos(kernel *k, double x)
 {
     double radius = k->radius;
-    assert(radius >= 1.0);
-
-    // This is already three orders of magnitude slower than anything you could
-    // possibly hope to play back in realtime and results in tons of ringing
-    // artifacts, so I doubt anybody will complain.
-    if (radius > 16)
-        radius = 16;
-
     if (fabs(x) < 1e-8)
         return 1.0;
     if (fabs(x) >= radius)
         return 0.0;
-
-    // Precomputed zeros of the jinc() function, needed to adjust the
-    // window size. Computing this at runtime is nontrivial.
-    // Copied from: https://github.com/AviSynth/jinc-resize/blob/master/JincResize/JincFilter.cpp#L171
-    static double jinc_zeros[16] = {
-        1.2196698912665045,
-        2.2331305943815286,
-        3.2383154841662362,
-        4.2410628637960699,
-        5.2427643768701817,
-        6.2439216898644877,
-        7.2447598687199570,
-        8.2453949139520427,
-        9.2458926849494673,
-        10.246293348754916,
-        11.246622794877883,
-        12.246898461138105,
-        13.247132522181061,
-        14.247333735806849,
-        15.247508563037300,
-        16.247661874700962
-    };
-
-    double window = jinc_zeros[0] / jinc_zeros[(int)radius - 1];
-    return jinc(k, x) * jinc(k, x*window);
+    // First zero of the jinc function. We simply scale it to fit into the
+    // given radius.
+    double jinc_zero = 1.2196698912665045;
+    return jinc(k, x) * jinc(k, x * jinc_zero / radius);
 }
 
 static double blackman(kernel *k, double x)