img_format: add mp_regular_imgfmt.forced_csp field

As the code comment says, this is needed to disambiguate FFmpeg formats.
This struct only describes the "physical" layout of a format, while
FFmpeg also attaches part of the colorspace information to the format.

video/img_format.c

@@ -454,6 +454,8 @@ bool mp_get_regular_imgfmt(struct mp_regular_imgfmt *dst, int imgfmt)
         return false; // it's satan himself
 #endif
 
+    res.forced_csp = mp_imgfmt_get_forced_csp(imgfmt);
+
     if (!validate_regular_imgfmt(&res))
         return false;
 

video/img_format.h

@@ -125,6 +125,11 @@ struct mp_regular_imgfmt {
     // Type of each component.
     enum mp_component_type component_type;
 
+    // See mp_imgfmt_get_forced_csp(). Normally code should use
+    // mp_image_params.colors. This field is only needed to map the format
+    // unambiguously to FFmpeg formats.
+    enum mp_csp forced_csp;
+
     // Size of each component in bytes.
     uint8_t component_size;