swscale/x86: use dedicated int for self-modifying MMX dstW

I want to pull options out of SwsInternal, so we need to make this field
a dedicated int that gets updated as appropriate in ff_swscale().

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

libswscale/swscale.c

@@ -488,6 +488,7 @@ int ff_swscale(SwsContext *c, const uint8_t *const src[], const int srcStride[],
 
 #if HAVE_MMX_INLINE
         ff_updateMMXDitherTables(c, dstY);
+        c->dstW_mmx = c->dstW;
 #endif
         if (should_dither) {
             c->chrDither8 = ff_dither_8x8_128[chrDstY & 7];

libswscale/swscale_internal.h

@@ -345,6 +345,7 @@ struct SwsContext {
     SwsFunc convert_unscaled;
     int srcW;                     ///< Width  of source      luma/alpha planes.
     int srcH;                     ///< Height of source      luma/alpha planes.
+    int dstW;                     ///< Width  of destination luma/alpha planes.
     int dstH;                     ///< Height of destination luma/alpha planes.
     int chrSrcW;                  ///< Width  of source      chroma     planes.
     int chrSrcH;                  ///< Height of source      chroma     planes.
@@ -534,7 +535,7 @@ struct SwsContext {
     DECLARE_ALIGNED(8, uint64_t, vOffset);
     int32_t lumMmxFilter[4 * MAX_FILTER_SIZE];
     int32_t chrMmxFilter[4 * MAX_FILTER_SIZE];
-    int dstW;                     ///< Width  of destination luma/alpha planes.
+    int dstW_mmx;
     DECLARE_ALIGNED(8, uint64_t, esp);
     DECLARE_ALIGNED(8, uint64_t, vRounder);
     DECLARE_ALIGNED(8, uint64_t, u_temp);