swscale: add sws_test_hw_format()

Allows users to test if a given hardware pixel format is supported by swscale.
This is only a rough heuristic anyways, because the actual support may
depend on the specific *combination* of frame attributes, and ultimately
is better served by the `sws_test_frame` and `sws_frame_setup()` APIs anyways.

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

doc/APIchanges

@@ -2,6 +2,9 @@ The last version increases of all libraries were on 2025-03-28
 
 API changes, most recent first:
 
+2026-02-xx - xxxxxxxxxx - lsws 9.4.100 - swscale.h
+  Add sws_test_hw_format().
+
 2026-02-19 - xxxxxxxxxx - lavf 62.10.100 - avformat.h
   Add demuxer command API consisting of:
     - Add avformat_send_command().

libswscale/format.c

@@ -527,6 +527,14 @@ int sws_test_format(enum AVPixelFormat format, int output)
     return output ? sws_isSupportedOutput(format) : sws_isSupportedInput(format);
 }
 
+int sws_test_hw_format(enum AVPixelFormat format)
+{
+    switch (format) {
+    case AV_PIX_FMT_NONE: return 1;
+    default: return 0;
+    }
+}
+
 int sws_test_colorspace(enum AVColorSpace csp, int output)
 {
     switch (csp) {

libswscale/swscale.h

@@ -267,7 +267,7 @@ void sws_free_context(SwsContext **ctx);
  ***************************/
 
 /**
- * Test if a given pixel format is supported.
+ * Test if a given (software) pixel format is supported.
  *
  * @param output  If 0, test if compatible with the source/input frame;
  *                otherwise, with the destination/output frame.
@@ -277,6 +277,15 @@ void sws_free_context(SwsContext **ctx);
  */
 int sws_test_format(enum AVPixelFormat format, int output);
 
+/**
+ * Test if a given hardware pixel format is supported.
+ *
+ * @param format  The hardware format to check, or AV_PIX_FMT_NONE.
+ *
+ * @return A positive integer if supported or AV_PIX_FMT_NONE, 0 otherwise.
+ */
+int sws_test_hw_format(enum AVPixelFormat format);
+
 /**
  * Test if a given color space is supported.
  *

libswscale/version.h

@@ -28,7 +28,7 @@
 
 #include "version_major.h"
 
-#define LIBSWSCALE_VERSION_MINOR   3
+#define LIBSWSCALE_VERSION_MINOR   4
 #define LIBSWSCALE_VERSION_MICRO 100
 
 #define LIBSWSCALE_VERSION_INT  AV_VERSION_INT(LIBSWSCALE_VERSION_MAJOR, \