display_driver: a direct panel reports each frame once, and presents counts it

[bdbarnett]

On a direct-framebuffer panel, display_driver now reports each frame once, whichever way the frame was presented. presents counts it either way.

_flush_cb_direct hands every dirty area to the panel's flush_rect. On a frame's last area it used to call show() and count a present only when flush_rect could not sync the area. A panel whose flush_rect does sync, like the P4's DSI panel through mipidsi's refresh_rect, never counted a present, so presents stayed at 0 there. Now the last area ends in show() when flush_rect didn't sync, or in the panel's frame_done() when it did, and presents += 1 in both cases.

frame_done() is the hook added to displaydev's DisplayDriver for its new frame-rate meter (PyDevices/pydevices#129). It does nothing unless measure_fps is on. A displaydev without it is fine too: the call is skipped by getattr.

I made the change here and not only in lvgl-micropython's vendored copy, which is where the brief put it. This is the canonical copy, so a sync would have overwritten an edit made only there, and lvgl-python, which gives CPython its display_driver and runs CastDisplay, would never have got it. The lvgl-micropython PR carries the identical file.

Measured (desktop, real LVGL, FBDisplay in DIRECT mode, two labels redrawn every 40 ms)

runtime	panel	frames (displaydev)	presents	flushes
CPython	buffer with refresh_rect (P4's branch)	124	124	249
CPython	plain buffer (CastDisplay's branch)	124	124	249
MicroPython unix	buffer with refresh_rect	125	125	251
MicroPython unix	plain buffer	124	124	249
CPython	CastDisplay running lv_test_timer	166	166	174

With the old display_driver, the refresh_rect panel reads frames 0 and presents 0 against 249 flushes.

tests/test_display_driver_direct_frames.py runs the real _flush_cb_direct against stand-in panels. Against the old code it fails the synced and no-frame_done cases.

After this merges, lvgl-micropython and lvgl-python need their usual sync.

[bdbarnett]

Closing in favour of the move: display_driver now lives in PyDevices/pydevices lib/, and the file that lands there is this PR's version, frame hook and all (PyDevices/pydevices#129). The tests here moved with it, to pydevices' tests/test_lvgl_display_driver.py. Removal from this repo is #25.