h264/frame.go: make frame type consts go-like

[saxon-milton]

h264decode/h264/frame.go

Lines 3 to 21 in f862767

	const (
	NALU_TYPE_UNSPECIFIED = iota
	NALU_TYPE_SLICE_NON_IDR_PICTURE
	NALU_TYPE_SLICE_PART_A
	NALU_TYPE_SLICE_PART_B
	NALU_TYPE_SLICE_PART_C
	NALU_TYPE_SLICE_IDR_PICTURE
	NALU_TYPE_SEI_SINFO
	NALU_TYPE_SPS
	NALU_TYPE_PPS
	NALU_TYPE_ACCESS_UNIT_DELIMITER
	NALU_TYPE_END_OF_SEQUENCE
	NALU_TYPE_END_OF_STREAM
	NALU_TYPE_FILLER_DATA
	NALU_TYPE_SPS_EXTENSIONS
	NALU_TYPE_PREFIX_NALU
	NALU_TYPE_SUBSET_SPS
	NALU_TYPE_DEPTH_PARAM_SET
	)

Make these go-like. Doing so will probably break other parts of code - remember to fix those too.