chroma-encoder: Add NTSC-specific chroma component filter

Change the filter used in the wideband-yuv/yiq modes to
one suggested by Clarke for NTSC.

Small PSNR improvement, up to 0.03 dB.

tools/ld-chroma-decoder/encoder/ntscencoder.cpp

@@ -268,15 +268,13 @@ static double syncPulseGate(double t, double startTime, SyncPulseType type)
     return raisedCosineGate(t, startTime, startTime + length, 200.0e-9 / 2.0);
 }
 
-// 1.3 MHz low-pass Gaussian filter, as used in pyctools-pal's coder.
-// Generated by: c = scipy.signal.gaussian(13, 1.49); c / sum(c)
+// 1.3 MHz low-pass filter
 //
-// The UV filter should be 0 dB at 0 Hz, >= -3 dB at 1.3 MHz, <= -20 dB at
-// 4.0 MHz. [Clarke p8]
-static constexpr std::array<double, 13> uvFilterCoeffs {
-    8.06454142158873e-05, 0.0009604748783110286, 0.007290763490157312, 0.035272860169480155, 0.10876496139131472,
-    0.21375585039760908, 0.2677488885178237, 0.21375585039760908, 0.10876496139131472, 0.035272860169480155,
-    0.007290763490157312, 0.0009604748783110286, 8.06454142158873e-05
+// The filter should be 0 dB at 0 Hz, >= -2 dB at 1.3 MHz, < -20 dB at
+// 3.6 MHz. [Clarke p15]
+static constexpr std::array<double, 9> uvFilterCoeffs {
+    0.0021, 0.0191, 0.0903, 0.2308, 0.3153,
+    0.2308, 0.0903, 0.0191, 0.0021
 };
 static constexpr auto uvFilter = makeFIRFilter(uvFilterCoeffs);