Add YUV420 riskiness metric.

Basic version for 8 bit only and C only (no simd).

Hidden behind a compile flag because the precompiled table is
fairly large (114kiB) and more than triples the size of the library.

Change-Id: I165c78e863df6a17b32f578bdbffe3adda9ac1d0

build.gradle

@@ -112,6 +112,7 @@ model {
             include "sharpyuv_dsp.c"
             include "sharpyuv_gamma.c"
             include "sharpyuv_neon.c"
+            include "sharpyuv_risk_table.c"
             include "sharpyuv_sse2.c"
             srcDir "src/dec"
             include "alpha_dec.c"