exr: make channel_offsets int instead of int8_t

Prior to this change max number of channels for float data which was
going to be correctly decoded was 32, which is rather small
considering that exr allows multiple channel layers.
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavcodec/exr.c

@@ -52,7 +52,7 @@ typedef struct EXRContext {
     AVFrame picture;
     int compr;
     int bits_per_color_id;
-    int8_t channel_offsets[4]; // 0 = red, 1 = green, 2 = blue and 3 = alpha
+    int channel_offsets[4]; // 0 = red, 1 = green, 2 = blue and 3 = alpha
 
     uint8_t *uncompressed_data;
     int uncompressed_size;