swscale/internal: use static_assert for enforcing offsets

Instead of sprinkling av_assert0 into random init functions.

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Niklas Haas <git@haasn.dev>

libswscale/swscale_internal.h

@@ -22,6 +22,7 @@
 #define SWSCALE_SWSCALE_INTERNAL_H
 
 #include <stdatomic.h>
+#include <assert.h>
 
 #include "config.h"
 #include "swscale.h"
@@ -705,6 +706,16 @@ struct SwsInternal {
 };
 //FIXME check init (where 0)
 
+static_assert(offsetof(SwsInternal, redDither) + DITHER32_INT == offsetof(SwsInternal, dither32),
+              "dither32 must be at the same offset as redDither + DITHER32_INT");
+
+#if ARCH_X86_64
+/* x86 yuv2gbrp uses the SwsInternal for yuv coefficients
+   if struct offsets change the asm needs to be updated too */
+static_assert(offsetof(SwsInternal, yuv2rgb_y_offset) == 40292,
+              "yuv2rgb_y_offset must be updated in x86 asm");
+#endif
+
 SwsFunc ff_yuv2rgb_get_func_ptr(SwsInternal *c);
 int ff_yuv2rgb_c_init_tables(SwsInternal *c, const int inv_table[4],
                              int fullRange, int brightness,

libswscale/utils.c

@@ -1228,8 +1228,6 @@ SwsContext *sws_alloc_context(void)
 {
     SwsInternal *c = av_mallocz(sizeof(SwsInternal));
 
-    av_assert0(offsetof(SwsInternal, redDither) + DITHER32_INT == offsetof(SwsInternal, dither32));
-
     if (c) {
         c->av_class = &ff_sws_context_class;
         av_opt_set_defaults(c);

libswscale/x86/swscale.c

@@ -791,10 +791,6 @@ switch(c->dstBpc){ \
 
     if(c->flags & SWS_FULL_CHR_H_INT) {
 
-        /* yuv2gbrp uses the SwsInternal for yuv coefficients
-           if struct offsets change the asm needs to be updated too */
-        av_assert0(offsetof(SwsInternal, yuv2rgb_y_offset) == 40292);
-
 #define YUV2ANYX_FUNC_CASE(fmt, name, opt)              \
         case fmt:                                       \
             c->yuv2anyX = ff_yuv2##name##_full_X_##opt; \