swscale/utils: Add check that ensures that the hardcoded struct offsets are valid

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/swscale_internal.h

@@ -429,6 +429,7 @@ typedef struct SwsContext {
 #define UV_OFF_BYTE           "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*3+56"
 #define DITHER16              "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*3+64"
 #define DITHER32              "11*8+4*4*"AV_STRINGIFY(MAX_FILTER_SIZE)"*3+80"
+#define DITHER32_INT          (11*8+4*4*MAX_FILTER_SIZE*3+80) // value equal to above, used for checking that the struct hasnt been changed by mistake
 
     DECLARE_ALIGNED(8, uint64_t, redDither);
     DECLARE_ALIGNED(8, uint64_t, greenDither);

libswscale/utils.c

@@ -1107,6 +1107,8 @@ SwsContext *sws_alloc_context(void)
 {
     SwsContext *c = av_mallocz(sizeof(SwsContext));
 
+    av_assert0(offsetof(SwsContext, redDither) + DITHER32_INT == offsetof(SwsContext, dither32));
+
     if (c) {
         c->av_class = &sws_context_class;
         av_opt_set_defaults(c);