Fix C++ build

With ImageInner::StaticTextures (the variant) and StaticTextures (the
struct), we need to disambiguate for the generated enum contructor
functions. This is done by prefixing the enum variant with their name.

api/cpp/cbindgen.rs

@@ -227,6 +227,7 @@ fn gen_corelib(
                 "slint_image_path",
                 "SharedPixelBuffer",
                 "SharedImageBuffer",
+                "StaticTextures",
             ],
             vec!["Color"],
             "slint_image_internal.h",