swscale: add SWS_UNSTABLE flag

Give users and developers a way to opt in to the new format conversion code,
and more code from the swscale rewrite in general, even while development is
still ongoing.

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2025-03-28
 
 API changes, most recent first:
 
+2025-09-xx - xxxxxxxxxx - lsws 9.3.100 - swscale.h
+  Add SWS_UNSTABLE flag.
+
 2025-09-01 - xxxxxxxx - lavc 62.14.100 - packet.h
   Add AV_PKT_DATA_EXIF
 

doc/scaler.texi

@@ -68,6 +68,10 @@ Select full chroma input.
 
 @item bitexact
 Enable bitexact output.
+
+@item unstable
+Allow the use of experimental new code. May subtly affect the output or even
+produce wrong results. For testing only.
 @end table
 
 @item srcw @var{(API only)}

libswscale/options.c

@@ -50,6 +50,7 @@ static const AVOption swscale_options[] = {
         { "full_chroma_inp", "full chroma input",             0,  AV_OPT_TYPE_CONST, { .i64 = SWS_FULL_CHR_H_INP }, .flags = VE, .unit = "sws_flags" },
         { "bitexact",        "bit-exact mode",                0,  AV_OPT_TYPE_CONST, { .i64 = SWS_BITEXACT       }, .flags = VE, .unit = "sws_flags" },
         { "error_diffusion", "error diffusion dither",        0,  AV_OPT_TYPE_CONST, { .i64 = SWS_ERROR_DIFFUSION}, .flags = VE, .unit = "sws_flags" },
+        { "unstable",        "allow experimental new code",   0,  AV_OPT_TYPE_CONST, { .i64 = SWS_UNSTABLE       }, .flags = VE, .unit = "sws_flags" },
 
     { "param0",          "scaler param 0", OFFSET(scaler_params[0]), AV_OPT_TYPE_DOUBLE, { .dbl = SWS_PARAM_DEFAULT  }, INT_MIN, INT_MAX, VE },
     { "param1",          "scaler param 1", OFFSET(scaler_params[1]), AV_OPT_TYPE_DOUBLE, { .dbl = SWS_PARAM_DEFAULT  }, INT_MIN, INT_MAX, VE },

libswscale/swscale.h

@@ -155,6 +155,13 @@ typedef enum SwsFlags {
     SWS_ACCURATE_RND   = 1 << 18,
     SWS_BITEXACT       = 1 << 19,
 
+    /**
+     * Allow using experimental new code paths. This may be faster, slower,
+     * or produce different output, with semantics subject to change at any
+     * point in time. For testing and debugging purposes only.
+     */
+    SWS_UNSTABLE = 1 << 20,
+
     /**
      * Deprecated flags.
      */

libswscale/version.h

@@ -28,7 +28,7 @@
 
 #include "version_major.h"
 
-#define LIBSWSCALE_VERSION_MINOR   2
+#define LIBSWSCALE_VERSION_MINOR   3
 #define LIBSWSCALE_VERSION_MICRO 100
 
 #define LIBSWSCALE_VERSION_INT  AV_VERSION_INT(LIBSWSCALE_VERSION_MAJOR, \