swscale: fix sws_setColorspaceDetails after sws_init_context

More commonly, this fixes the case of sws_setColorspaceDetails after
sws_getContext, since the latter implies sws_init_context.

The problem here is that sws_init_context sets up the range conversion
and fast path tables based on the values of srcRange/dstRange at init
time. This may result in locking in a "wrong" path (either using
unscaled fast path when range conversion later required, or using
scaled slow path when range conversion becomes no longer required).

There are two way outs:

1. Always initialize range conversion and unscaled converters, even if
   they will be unused, and extend the runtime check.
2. Re-do initialization if the values change after
   sws_setColorspaceDetails.

I opted for approach 1 because it was simpler and easier to reason
about.

Reword the av_log message to make it clear that this special converter
is not necessarily used, depending on whether or not there is range
conversion or YUV matrix conversion going on.

libswscale/swscale.c

@@ -1016,7 +1016,7 @@ static int scale_internal(SwsContext *c,
     reset_ptr(src2, c->srcFormat);
     reset_ptr((void*)dst2, c->dstFormat);
 
-    if (c->convert_unscaled) {
+    if (c->convert_unscaled && !c->lumConvertRange && !c->chrConvertRange) {
         int offset  = srcSliceY_internal;
         int slice_h = srcSliceH;
 

libswscale/utils.c

@@ -1715,30 +1715,26 @@ static av_cold int sws_init_single_context(SwsContext *c, SwsFilter *srcFilter,
     if (unscaled && !usesHFilter && !usesVFilter &&
         c->alphablend != SWS_ALPHA_BLEND_NONE &&
         isALPHA(srcFormat) &&
-        (c->srcRange == c->dstRange || isAnyRGB(dstFormat)) &&
         alphaless_fmt(srcFormat) == dstFormat
     ) {
         c->convert_unscaled = ff_sws_alphablendaway;
 
         if (flags & SWS_PRINT_INFO)
             av_log(c, AV_LOG_INFO,
-                    "using alpha blendaway %s -> %s special converter\n",
+                    "alpha blendaway %s -> %s special converter is available\n",
                     av_get_pix_fmt_name(srcFormat), av_get_pix_fmt_name(dstFormat));
         return 0;
     }
 
     /* unscaled special cases */
-    if (unscaled && !usesHFilter && !usesVFilter &&
-        (c->srcRange == c->dstRange || isAnyRGB(dstFormat) ||
-         isFloat(srcFormat) || isFloat(dstFormat))){
+    if (unscaled && !usesHFilter && !usesVFilter) {
         ff_get_unscaled_swscale(c);
 
         if (c->convert_unscaled) {
             if (flags & SWS_PRINT_INFO)
                 av_log(c, AV_LOG_INFO,
-                       "using unscaled %s -> %s special converter\n",
+                       "unscaled %s -> %s special converter is available\n",
                        av_get_pix_fmt_name(srcFormat), av_get_pix_fmt_name(dstFormat));
-            return 0;
         }
     }