Fix periodic intra frames and feature reset for chunk-based models

[DucTu (DucTu2k4)]

Problem

With --model_structure hts (or htl) and a periodic intra setting such as
--force_intra_period 128, test_video.py inserts only one periodic intra frame
and then stops both intra insertion and feature-memory reset for the rest of the
sequence.

In run_one_point_with_stream, an intra frame is inserted when
frame_idx % intra_period == 1, checked at the start of each chunk. With
g_frame_delay = 8 the chunk starts are 1, 9, ..., 129, so frame 129 is hit.
The intra frame consumes a single frame, so the following chunks start at
130, 138, ... and the condition is never true again. The reset condition
(frame_idx + g_frame_delay) % reset_interval == 1 is affected in the same way.

For a 750-frame sequence this gives intra frames at [0, 129] only, and the last
620 frames are coded without any intra frame or reset.

Fix

• Insert an intra frame when the GOP index max(frame_idx - 1, 0) // intra_period
  increases, instead of testing frame_idx directly.
• Trigger the reset when the current chunk covers a multiple of reset_interval.

Behaviour is unchanged for the LD model (g_frame_delay = 1) and for HT models
without periodic intra frames (intra_period = -1); I checked this by simulating
the loop for several frame counts, intra periods and reset intervals.

Note: since an intra frame shifts the chunk alignment by one frame, the distance
between intra frames is intra_period + 1 for most GOPs (0, 129, 258, 387, ...),
and one GOP is shortened by g_frame_delay every g_frame_delay GOPs so that the
average period stays intra_period. I am happy to change this if you prefer a
different convention.

Results

HT-S, QP 40, 1920x1080, 750 frames, 8-bit YUV420, --reset_interval 128,
--skip_thres 0.15, RTX 3090.

Setting	I frames	Avg bpp	P-frame PSNR-Y
main, --force_intra_period 128	2	0.0740	25.01 dB
This PR, --force_intra_period 128	6	0.0040	41.59 dB
Reference, intra_period = -1	1	0.0034	41.50 dB