avutil/half2float: adjust conversion of NaN

IEEE-754 differentiates two different kind of NaNs.
Quiet and Signaling ones. They are differentiated by the MSB of the
mantissa.

For whatever reason, actual hardware conversion of half to single always
sets the signaling bit to 1 if the mantissa is != 0, and to 0 if it's 0.
So our code has to follow suite or fate-testing hardware float16 will be
impossible.

libavcodec/exr.c

@@ -191,7 +191,7 @@ typedef struct EXRContext {
     float gamma;
     union av_intfloat32 gamma_table[65536];
 
-    uint32_t mantissatable[2048];
+    uint32_t mantissatable[3072];
     uint32_t exponenttable[64];
     uint16_t offsettable[64];
 } EXRContext;