avcodec/snow: Reduce sizeof(SnowContext)

Each SubBand currently contains an array of 519 uint8_t[32],
yet most of these are unused: For both the decoder and the
encoder, at most 34 contexts are actually used: The only
variable index is context+2, where context is the result
of av_log2() and therefore in the 0..31 range.

There are also several accesses using compile-time indices,
the highest of which is 30. FATE passes with 31 contexts
and maybe these are enough, but I don't know.

Reducing the number to 34 reduces sizeof(SnowContext)
from 2141664B to 155104B here (on x64).

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/snow.h

@@ -92,7 +92,7 @@ typedef struct SubBand{
     int stride_line; ///< Stride measured in lines, not pixels.
     x_and_coeff * x_coeff;
     struct SubBand *parent;
-    uint8_t state[/*7*2*/ 7 + 512][32];
+    uint8_t state[34][32];
 }SubBand;
 
 typedef struct Plane{