hwcontext: Make it easier to work with device types

Adds functions to convert to/from strings and a function to iterate
over all supported device types.  Also adds a new invalid type
AV_HWDEVICE_TYPE_NONE, which acts as a sentinel value.

libavutil/hwcontext.c

@@ -47,6 +47,47 @@ static const HWContextType * const hw_table[] = {
     NULL,
 };
 
+const char *hw_type_names[] = {
+    [AV_HWDEVICE_TYPE_CUDA]   = "cuda",
+    [AV_HWDEVICE_TYPE_DXVA2]  = "dxva2",
+    [AV_HWDEVICE_TYPE_QSV]    = "qsv",
+    [AV_HWDEVICE_TYPE_VAAPI]  = "vaapi",
+    [AV_HWDEVICE_TYPE_VDPAU]  = "vdpau",
+};
+
+enum AVHWDeviceType av_hwdevice_find_type_by_name(const char *name)
+{
+    int type;
+    for (type = 0; type < FF_ARRAY_ELEMS(hw_type_names); type++) {
+        if (hw_type_names[type] && !strcmp(hw_type_names[type], name))
+            return type;
+    }
+    return AV_HWDEVICE_TYPE_NONE;
+}
+
+const char *av_hwdevice_get_type_name(enum AVHWDeviceType type)
+{
+    if (type >= 0 && type < FF_ARRAY_ELEMS(hw_type_names))
+        return hw_type_names[type];
+    else
+        return NULL;
+}
+
+enum AVHWDeviceType av_hwdevice_iterate_types(enum AVHWDeviceType prev)
+{
+    enum AVHWDeviceType next;
+    int i, set = 0;
+    for (i = 0; hw_table[i]; i++) {
+        if (prev != AV_HWDEVICE_TYPE_NONE && hw_table[i]->type <= prev)
+            continue;
+        if (!set || hw_table[i]->type < next) {
+            next = hw_table[i]->type;
+            set = 1;
+        }
+    }
+    return set ? next : AV_HWDEVICE_TYPE_NONE;
+}
+
 static const AVClass hwdevice_ctx_class = {
     .class_name = "AVHWDeviceContext",
     .item_name  = av_default_item_name,