linux-v4l2: Add 'Use System Timing' property

Due to potential driver issues with certain devices, the timestamps are
not always reliable.  This option allows of using the time in which the
frame was received as a timestamp instead.

plugins/linux-v4l2/data/locale/en-US.ini

@@ -5,3 +5,4 @@ ImageFormat="Video Format"
 Resolution="Resolution"
 FrameRate="Frame Rate"
 LeaveUnchanged="Leave Unchanged"
+UseSystemTiming="Use System Timing"

plugins/linux-v4l2/v4l2-input.c

@@ -32,6 +32,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 #include <util/threading.h>
 #include <util/bmem.h>
 #include <util/dstr.h>
+#include <util/platform.h>
 #include <obs-module.h>
 
 #include "v4l2-helpers.h"
@@ -57,6 +58,7 @@ struct v4l2_data {
 	int pixfmt;
 	int resolution;
 	int framerate;
+	bool sys_timing;
 
 	/* internal data */
 	obs_source_t *source;
@@ -177,7 +179,9 @@ static void *v4l2_thread(void *vptr)
 			break;
 		}
 
-		out.timestamp = timeval2ns(buf.timestamp);
+		out.timestamp = data->sys_timing ?
+			os_gettime_ns() : timeval2ns(buf.timestamp);
+
 		start = (uint8_t *) data->buffers.info[buf.index].start;
 		for (uint_fast32_t i = 0; i < MAX_AV_PLANES; ++i)
 			out.data[i] = start + plane_offsets[i];
@@ -209,6 +213,7 @@ static void v4l2_defaults(obs_data_t *settings)
 	obs_data_set_default_int(settings, "pixelformat", -1);
 	obs_data_set_default_int(settings, "resolution", -1);
 	obs_data_set_default_int(settings, "framerate", -1);
+	obs_data_set_default_bool(settings, "system_timing", false);
 }
 
 /*
@@ -576,6 +581,9 @@ static obs_properties_t *v4l2_properties(void *unused)
 			"framerate", obs_module_text("FrameRate"),
 			OBS_COMBO_TYPE_LIST, OBS_COMBO_FORMAT_INT);
 
+	obs_properties_add_bool(props,
+			"system_timing", obs_module_text("UseSystemTiming"));
+
 	v4l2_device_list(device_list, NULL);
 	obs_property_set_modified_callback(device_list, device_selected);
 	obs_property_set_modified_callback(input_list, input_selected);
@@ -711,6 +719,7 @@ static void v4l2_update(void *vptr, obs_data_t *settings)
 	data->pixfmt     = obs_data_get_int(settings, "pixelformat");
 	data->resolution = obs_data_get_int(settings, "resolution");
 	data->framerate  = obs_data_get_int(settings, "framerate");
+	data->sys_timing = obs_data_get_bool(settings, "system_timing");
 
 	v4l2_init(data);
 }